[gimp] Fix build on osx by adding appkit dependency

[Michael Natterer <mitch src gnome org>]

commit f464ea0d4e3b974b50e32ed24b52783ad65ebdb5
Author: Alex Samorukov <samm os2 kiev ua>
Date:   Fri Jun 8 09:58:08 2018 +0000

    Fix build on osx by adding appkit dependency

 app-tools/Makefile.am | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/app-tools/Makefile.am b/app-tools/Makefile.am
index d0f6e9445d..75200b1e90 100644
--- a/app-tools/Makefile.am
+++ b/app-tools/Makefile.am
@@ -9,6 +9,7 @@ if PLATFORM_OSX
 xobjective_c = "-xobjective-c"
 xobjective_cxx = "-xobjective-c++"
 xnone = "-xnone"
+framework_appkit = -framework AppKit
 endif
 
 if OS_WIN32
@@ -78,4 +79,5 @@ AM_CXXFLAGS = \
        $(xobjective_cxx)
 
 AM_LDFLAGS = \
+       $(framework_appkit)     \
        $(xnone)