inspector: Fix the build

I forgot to test the previous commit with -Wimplicit-function-delcaration,
and it promptly broke.

gtk/inspector/general.c

@@ -20,7 +20,10 @@
 
 #include "general.h"
 
+#include "gtkadjustment.h"
 #include "gtkdebug.h"
+#include "gtklabel.h"
+#include "gtkscale.h"
 #include "gtkswitch.h"
 #include "gtklistbox.h"
 #include "gtkprivate.h"