sox: fixes Homebrew/homebrew#2312

mp3.c:60: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘attribute’ before ‘const’
mp3.c:60: static mad_timer_t const mad_timer_zero_stub = {0, 0};
uses mad data type outside ifdef MAD, should be fixed upstream

Formula/sox.rb

@@ -9,6 +9,7 @@ class Sox <Formula
   depends_on 'libvorbis' => :optional
   depends_on 'flac' => :optional
   depends_on 'libao' => :optional
+  depends_on 'mad' # see commit message
 
   def install
     system "./configure", "--disable-debug", "--disable-dependency-tracking",