[libgxps] Fix the build with libpng16

[Carlos Garcia Campos <carlosgc src gnome org>]

commit d469b385f7c91b41e51a7a274926e14e9c2963be
Author: Petr Gajdos <pgajdos suse cz>
Date:   Mon Feb 18 17:16:05 2013 +0100

    Fix the build with libpng16
    
    Include string.h explicitly in gxps-png-writer.c since png.h no longer
    includes it.

 tools/gxps-png-writer.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/tools/gxps-png-writer.c b/tools/gxps-png-writer.c
index 369e0e0..263b8ae 100644
--- a/tools/gxps-png-writer.c
+++ b/tools/gxps-png-writer.c
@@ -22,6 +22,7 @@
 #include "gxps-png-writer.h"
 #include <png.h>
 #include <stdint.h>
+#include <string.h>
 
 /* starting with libpng15, png.h no longer #includes zlib.h */
 #ifndef Z_BEST_COMPRESSION