[gitg] Fix libgitg compile on macOS

[Alberto Fanjul <albfan src gnome org>]

commit 67f5cd6925e8bf1e4c7e5b65fe9370c2cdd1d273
Author: Bo Anderson <mail boanderson me>
Date:   Mon Mar 30 14:07:18 2020 +0000

    Fix libgitg compile on macOS

 libgitg/gitg-platform-support-osx.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/libgitg/gitg-platform-support-osx.c b/libgitg/gitg-platform-support-osx.c
index 088170cf..b90afe2e 100644
--- a/libgitg/gitg-platform-support-osx.c
+++ b/libgitg/gitg-platform-support-osx.c
@@ -23,6 +23,7 @@
 
 #include "gitg-platform-support.h"
 
+#import <AppKit/AppKit.h>
 #include <gdk/gdkquartz.h>
 #include <gio/gunixinputstream.h>