warning fix from a patch by Gustavo Sverzut Barbieri, barbieri gmail com

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@6843 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

vhook/drawtext.c

@@ -94,7 +94,7 @@
 
 typedef struct {
   unsigned char *text;
-  unsigned char *file;
+  char *file;
   unsigned int x;
   unsigned int y;
   int bg;