avcodec: Allow usage of VLC_CODEC_RGBA

PIX_FMT_RGBA is not a define, but an enum member.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
(cherry picked from commit c23388a9)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/avcodec/chroma.c

@@ -105,9 +105,7 @@ static const struct
     VLC_RGB( VLC_CODEC_RGB32, PIX_FMT_RGB32, PIX_FMT_BGR32, 0x00ff0000, 0x0000ff00, 0x000000ff )
     VLC_RGB( VLC_CODEC_RGB32, PIX_FMT_RGB32_1, PIX_FMT_BGR32_1, 0xff000000, 0x00ff0000, 0x0000ff00 )
 
-#if defined(PIX_FMT_RGBA)
     {VLC_CODEC_RGBA, PIX_FMT_RGBA, 0xff000000, 0x00ff0000, 0x0000ff00},
-#endif
     {VLC_CODEC_GREY, PIX_FMT_GRAY8, 0, 0, 0},
 
      /* Paletized RGB */