configure: fix VP7 standalone build

Found-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -2116,6 +2116,7 @@ vp5_decoder_select="h264chroma hpeldsp videodsp vp3dsp"
 vp6_decoder_select="h264chroma hpeldsp huffman videodsp vp3dsp"
 vp6a_decoder_select="vp6_decoder"
 vp6f_decoder_select="vp6_decoder"
+vp7_decoder_select="h264pred videodsp"
 vp8_decoder_select="h264pred videodsp"
 vp9_decoder_select="videodsp"
 webp_decoder_select="vp8_decoder"