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

Originally committed as revision 6843 to svn://svn.ffmpeg.org/ffmpeg/trunk

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;