Make the id3v1_genre_str array const, not just the strings it points to.

Originally committed as revision 14942 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mp3.c

@@ -30,7 +30,7 @@
 
 #define ID3v1_GENRE_MAX 125
 
-static const char *id3v1_genre_str[ID3v1_GENRE_MAX + 1] = {
+static const char * const id3v1_genre_str[ID3v1_GENRE_MAX + 1] = {
     [0] = "Blues",
     [1] = "Classic Rock",
     [2] = "Country",