signal/cbindgen.toml: add stdint.h to sys_includes for signalfd_siginfo types

The signalfd_siginfo struct uses uint32_t, int32_t, uint64_t, uint16_t
which require <stdint.h>. Without it, gnulib-based packages (m4, bison,
flex) fail configure's sigset_t test because signal.h doesn't compile.

src/header/signal/cbindgen.toml

@@ -6,7 +6,7 @@
 # - "pid_t As described in <sys/types.h>."
 # - "The <signal.h> header shall define the pthread_attr_t type as described in <sys/types.h>."
 # - "Inclusion of the <signal.h> header may make visible all symbols from the <time.h> header."
-sys_includes = ["sys/types.h"]
+sys_includes = ["sys/types.h", "stdint.h"]
 include_guard = "_RELIBC_SIGNAL_H"
 after_includes = """
 #include <bits/sigset-t.h> // for sigset_t