sys/un.h: define SUN_LEN

SUN_LEN is not in the POSIX <sys/un.h> spec but is a long-standing
BSD/glibc extension that portable AF_UNIX socket code depends on. KWin's
src/helpers/wayland_wrapper/wl-socket.c calls it directly and fails with

  error: implicit declaration of function 'SUN_LEN'

Definition matches glibc: offset of sun_path plus strlen of the pathname
it holds, i.e. the byte count to pass to bind()/connect() for an AF_UNIX
address. Pulls in <string.h> for strlen.

src/header/sys_un/cbindgen.toml

@@ -4,6 +4,22 @@
 # - "The <sys/un.h> header shall define the sa_family_t type as described in <sys/socket.h>."
 after_includes = """
 #include <bits/safamily-t.h> // for sa_family_t from sys/socket.h
+#include <string.h> // for strlen() used by SUN_LEN
+
+/*
+ * SUN_LEN() is not in the POSIX <sys/un.h> spec, but it is a long-standing
+ * BSD/glibc extension that portable Wayland/X11 socket code relies on:
+ * KWin's src/helpers/wayland_wrapper/wl-socket.c calls it directly and
+ * otherwise fails with
+ *   error: implicit declaration of function 'SUN_LEN'
+ * The definition matches glibc's: offset of sun_path plus the length of the
+ * pathname it holds, giving the number of bytes to pass to bind()/connect()
+ * for an AF_UNIX address.
+ */
+#ifndef SUN_LEN
+#define SUN_LEN(ptr) \\
+    ((size_t)(((struct sockaddr_un *)0)->sun_path) + strlen((ptr)->sun_path))
+#endif
 """
 include_guard = "_RELIBC_SYS_UN_H"
 language = "C"