monitor-manager: Don't include generated header in header

This tends to result in build system race conditions.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2555>

src/backends/meta-backend.c

@@ -71,6 +71,7 @@
 #include "meta/main.h"
 #include "meta/meta-backend.h"
 #include "meta/meta-context.h"
+#include "meta/meta-enum-types.h"
 #include "meta/util.h"
 
 #ifdef HAVE_PROFILER

src/backends/meta-monitor-manager-private.h

@@ -38,7 +38,6 @@
 #include "backends/meta-viewport-info.h"
 #include "core/util-private.h"
 #include "meta/display.h"
-#include "meta/meta-enum-types.h"
 #include "meta/meta-monitor-manager.h"
 
 #define META_MONITOR_MANAGER_MIN_SCREEN_WIDTH 640