lavc/tiff_common: add const to silent warnings

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/tiff_common.c

@@ -75,7 +75,7 @@ unsigned ff_tget(GetByteContext *gb, int type, int le)
     }
 }
 
-static char *auto_sep(int count, char *sep, int i, int columns)
+static const char *auto_sep(int count, const char *sep, int i, int columns)
 {
     if (sep)
         return i ? sep : "";