headers: give timerfd/signalfd prototypes C linkage

sys/timerfd.h and sys/signalfd.h declare their functions in the cbindgen
`trailer`, which is emitted after the include guard closes and OUTSIDE the
extern "C" block that cpp_compat=true generates. The prototypes therefore took
C++ linkage, so a C++ consumer asked the linker for a mangled symbol while
relibc exports the plain C one:

  alignedtimer.cpp:(.text+0x29a): undefined reference to
      `timerfd_create(int, int)'

(plasma-workspace libclock). C callers were unaffected, which is why this
survived until a C++ consumer appeared.

The prototypes are hand-written rather than generated on purpose: cbindgen
renders them with a bare `itimerspec` and a `clockid_t` parameter, which does
not resolve in a plain C99 include and previously broke libwayland's
TFD_CLOEXEC probe. That stays; only the linkage is corrected.

signalfd has no in-tree C++ consumer yet and is fixed pre-emptively.

src/header/sys_signalfd/cbindgen.toml

@@ -9,8 +9,24 @@ trailer = """
 #define SFD_NONBLOCK 0x800
 #endif
 
+/* extern "C" is REQUIRED here; cpp_compat only wraps cbindgen's GENERATED
+ * body, and this trailer is emitted outside it. Without this a C++ consumer
+ * links against the mangled name while relibc exports the plain C symbol.
+ * Same defect as sys_timerfd, which surfaced as
+ *   undefined reference to `timerfd_create(int, int)'
+ * once plasma-workspace (C++) used it. Fixed here pre-emptively: no in-tree
+ * C++ consumer of signalfd exists yet, so this is latent rather than observed.
+ */
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 int signalfd(int fd, const sigset_t *mask, size_t masksize);
 int signalfd4(int fd, const sigset_t *mask, size_t masksize, int flags);
+
+#ifdef __cplusplus
+}
+#endif
 """
 language = "C"
 style = "Tag"

src/header/sys_timerfd/cbindgen.toml

@@ -25,9 +25,30 @@ trailer = """
 #define TFD_TIMER_CANCEL_ON_SET 0x2
 #endif
 
+/* extern "C" is REQUIRED here and cannot be left to cpp_compat.
+ *
+ * cpp_compat=true wraps only the body cbindgen GENERATES. These three
+ * prototypes are hand-written in this trailer (see the [export] note below),
+ * and the trailer is emitted after the include guard closes, outside that
+ * wrapper. Without an explicit extern "C" they take C++ linkage, so a C++
+ * consumer asks the linker for the mangled name while relibc exports the plain
+ * C symbol:
+ *   alignedtimer.cpp:(.text+0x29a): undefined reference to
+ *       `timerfd_create(int, int)'
+ * (first hit: plasma-workspace libclock/alignedtimer.cpp). C callers were
+ * unaffected, which is why this survived until a C++ consumer appeared.
+ */
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 int timerfd_create(int clockid, int flags);
 int timerfd_settime(int fd, int flags, const struct itimerspec *new_value, struct itimerspec *old_value);
 int timerfd_gettime(int fd, struct itimerspec *curr_value);
+
+#ifdef __cplusplus
+}
+#endif
 """
 language = "C"
 style = "Tag"