mutter: fix undefined reference to gettid

Our musl package has a patch that should have made this patch unnecessary,
but it doesn't seem to work properly on armv6l-musl and armv7l-musl.
This should be investigated and properly fixed.

srcpkgs/mutter/patches/gettid.patch

@@ -0,0 +1,23 @@
+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);