Revert "mutter: fix undefined reference to gettid"

This patch was temporarily added to resolve a build failure that required
rebuilding the musl cross compilers.

This reverts commit d9e7fef13f.

srcpkgs/mutter/patches/gettid.patch

@@ -1,23 +0,0 @@
-gettid wasn't implemented in musl until 1.2.2+
-
-diff --git a/src/backends/native/meta-backend-native.c b/src/backends/native/meta-backend-native.c
-index 2245387..2842593 100644
---- a/src/backends/native/meta-backend-native.c
-+++ b/src/backends/native/meta-backend-native.c
-@@ -39,6 +39,7 @@
- #include "backends/native/meta-input-thread.h"
- 
- #include <stdlib.h>
-+#include <syscall.h>
- 
- #include "backends/meta-cursor-tracker-private.h"
- #include "backends/meta-idle-manager.h"
-@@ -231,7 +232,7 @@ meta_backend_native_post_init (MetaBackend *backend)
- 
-           priority = sched_get_priority_min (SCHED_RR);
-           meta_dbus_realtime_kit1_call_make_thread_realtime_sync (rtkit_proxy,
--                                                                  gettid (),
-+                                                                  (pid_t) syscall (SYS_gettid),
-                                                                   priority,
-                                                                   NULL,
-                                                                   &error);