Include <string.h> for memset, to avoid an implicit function
declaration and build failures with future compilers.

Fixed upstream as part of:

r5451 | gmerlin | 2018-11-27 20:38:42 +0100 (Tue, 27 Nov 2018) | 2 lines

* Update

diff --git a/src/fill_test.c b/src/fill_test.c
index 1d362bea4af85745..6fd97dab9e192baa 100644
--- a/src/fill_test.c
+++ b/src/fill_test.c
@@ -23,6 +23,7 @@
 //#include "colorspace.h" // Common routines
 #include <stdio.h>
 #include <png.h>
+#include <string.h>
 
 static void
 write_png(char * filename, gavl_video_format_t * format, gavl_video_frame_t * frame)