forward: Drop socklen_t forward declaration

This is glibc specific and doesn't exist on musl, since removing only
means adding one more include in selinux-util.h, let's drop it.

Fixes #37779

src/basic/forward.h

@@ -57,7 +57,6 @@ struct ucred;
  * used for C++ symbol name mangling, they're effectively part of the ABI and won't actually change. */
 typedef struct _IO_FILE FILE;
 typedef struct __dirstream DIR;
-typedef __socklen_t socklen_t;
 
 /* 3rd-party library forward declarations */