Everywhere: Refer to pthread.h by its non-prefixed name

This removes a bit of noise from the following patches, where we will
move the `pthread.h` header out of the `LibPthread` directory.

Userland/Libraries/LibC/bits/pthread_forward.h

@@ -6,7 +6,7 @@
 
 #pragma once
 
-#include <LibPthread/pthread.h>
+#include <pthread.h>
 
 struct PthreadFunctions {
     int (*pthread_mutex_trylock)(pthread_mutex_t* mutex);