move misplaced __fork_handler declaration

pthread_atfork.c does not actually include pthread_impl.h and has no
reason to, so it wasn't getting the declaration. move it to libc.h
which is already included by both fork.c and pthread_atfork.c. this
makes more sense anyway since the function has little to do with
pthreads anyway aside from the name.

src/internal/libc.h

@@ -43,6 +43,7 @@ hidden void __libc_start_init(void);
 hidden void __funcs_on_exit(void);
 hidden void __funcs_on_quick_exit(void);
 hidden void __libc_exit_fini(void);
+hidden void __fork_handler(int);
 
 extern hidden size_t __hwcap;
 extern hidden size_t __sysinfo;

src/internal/pthread_impl.h

@@ -143,7 +143,6 @@ hidden void *__copy_tls(unsigned char *);
 hidden void __reset_tls();
 
 hidden void __dl_thread_cleanup(void);
-hidden void __fork_handler(int);
 hidden void __testcancel();
 hidden void __do_cleanup_push(struct __ptcb *);
 hidden void __do_cleanup_pop(struct __ptcb *);