Fix tests to compile again - add missing headers to W32, call correct process-id-getting function

https://bugzilla.gnome.org/show_bug.cgi?id=725513

gio/tests/gsubprocess-testprog.c

@@ -7,6 +7,8 @@
 #include <unistd.h>
 #include <gio/gunixinputstream.h>
 #include <gio/gunixoutputstream.h>
+#else
+#include <io.h>
 #endif
 
 static GOptionEntry options[] = {