[gimp] Fixing error on Windors meson build

[Michael Schumacher <schumaml src gnome org>]

commit a2b74147c9bdcf6ee03c6ed8049254ef47b6e810
Author: lillolollo <4179-lillolollo users noreply gitlab gnome org>
Date:   Wed Sep 11 17:21:17 2019 +0000

    Fixing error on Windors meson build

 app/plug-in/gimppluginmanager-file.c | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/app/plug-in/gimppluginmanager-file.c b/app/plug-in/gimppluginmanager-file.c
index fc4b0666df..07076e86d8 100644
--- a/app/plug-in/gimppluginmanager-file.c
+++ b/app/plug-in/gimppluginmanager-file.c
@@ -42,6 +42,10 @@
 
 #include "gimp-intl.h"
 
+#ifdef G_OS_WIN32
+#include <stdlib.h>
+#endif
+
 
 typedef enum
 {