app: fix #include order in gimp-spawn.c

... so that GLib is included before the platform-specific headers,
so that we can check for G_OS_WIN32.

Spotted by Partha, as usual :)

app/core/gimp-spawn.c

@@ -20,6 +20,8 @@
 
 #include "config.h"
 
+#include <glib-object.h>
+
 #ifdef HAVE_VFORK
 #include <sys/types.h>
 #include <sys/wait.h>
@@ -36,8 +38,6 @@
 #include <io.h>
 #endif
 
-#include <glib-object.h>
-
 #include "core-types.h"
 
 #include "gimp-spawn.h"