[gimp] app: add include to fix a warning.

[Jehan Pagès <jehanp src gnome org>]

commit 8120694759e47a5071113a7813efbb8307600ef6
Author: Jehan <jehan girinstud io>
Date:   Sat Jan 20 22:12:02 2018 +0100

    app: add include to fix a warning.
    
    The build warning was:
    > warning: implicit declaration of function ‘memcmp’ [-Wimplicit-function-declaration]

 app/display/gimpcanvastransformpreview.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/app/display/gimpcanvastransformpreview.c b/app/display/gimpcanvastransformpreview.c
index 7786c05..2b7cfca 100644
--- a/app/display/gimpcanvastransformpreview.c
+++ b/app/display/gimpcanvastransformpreview.c
@@ -20,6 +20,8 @@
 
 #include "config.h"
 
+#include <string.h>
+
 #include <gegl.h>
 #include <gtk/gtk.h>