testsuite: Fix header #include order on Windows

<Rts.h> must always come first.

testsuite/tests/rts/linker/unload_multiple_objs/linker_unload_multiple_objs.c

+#include "Rts.h"
 #include "ghcconfig.h"
+#include "HsFFI.h"
+
 #include <stdio.h>
 #include <stdlib.h>
-#include "Rts.h"
 #include <string.h>
-#include "HsFFI.h"
 
 extern void loadPackages(void);