build: Add missing sources for the GdkX11 gir build

Noticed while diffing .gir files with the meson port.

gdk/Makefile.am

@@ -274,8 +274,10 @@ x11_introspection_files = 		\
 	x11/gdkeventsource.c		\
 	x11/gdkeventtranslator.c	\
 	x11/gdkgeometry-x11.c		\
+	x11/gdkglcontext-x11.c		\
 	x11/gdkkeys-x11.c		\
 	x11/gdkmain-x11.c		\
+	x11/gdkmonitor-x11.c		\
 	x11/gdkproperty-x11.c		\
 	x11/gdkscreen-x11.c		\
 	x11/gdkselection-x11.c		\
@@ -298,7 +300,9 @@ x11_introspection_files = 		\
 	x11/gdkx11display.h		\
 	x11/gdkx11displaymanager.h	\
 	x11/gdkx11dnd.h			\
+	x11/gdkx11glcontext.h		\
 	x11/gdkx11keys.h		\
+	x11/gdkx11monitor.h		\
 	x11/gdkx11property.h		\
 	x11/gdkx11screen.h		\
 	x11/gdkx11selection.h		\