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.

Tests/Kernel/kill-pidtid-confusion.cpp

@@ -6,7 +6,7 @@
 
 #include <AK/Assertions.h>
 #include <AK/Format.h>
-#include <LibPthread/pthread.h>
+#include <pthread.h>
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>

Tests/LibPthread/TestLibPthreadCleanup.cpp

@@ -4,8 +4,8 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include <LibPthread/pthread.h>
 #include <LibTest/TestCase.h>
+#include <pthread.h>
 
 static size_t exit_count = 0;
 

Tests/LibPthread/TestLibPthreadRWLocks.cpp

@@ -4,8 +4,8 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include <LibPthread/pthread.h>
 #include <LibTest/TestCase.h>
+#include <pthread.h>
 
 TEST_CASE(rwlock_init)
 {

Tests/LibPthread/TestLibPthreadSpinLocks.cpp

@@ -4,9 +4,9 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include <LibPthread/pthread.h>
 #include <LibTest/TestCase.h>
 #include <errno.h>
+#include <pthread.h>
 #include <unistd.h>
 
 TEST_CASE(spin_init_process_scope)