configure: Make the smoothstreaming muxer enable the ismv muxer

This doesn't fix any build failure, but the smoothstreaming muxer
opens a chained ismv muxer.
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -1618,6 +1618,7 @@ rtsp_muxer_select="rtp_muxer http_protocol rtp_protocol"
 sap_demuxer_select="sdp_demuxer"
 sap_muxer_select="rtp_muxer rtp_protocol"
 sdp_demuxer_select="rtpdec"
+smoothstreaming_muxer_select="ismv_muxer"
 spdif_muxer_select="aac_parser"
 tg2_muxer_select="mov_muxer"
 tgp_muxer_select="mov_muxer"