configure: Let the libx264rgb encoder select the libx264 encoder.

Standalone compilation of the libx264rgb encoder does not work.

configure

@@ -2391,6 +2391,7 @@ libwavpack_encoder_deps="libwavpack"
 libwebp_encoder_deps="libwebp"
 libx264_encoder_deps="libx264"
 libx264rgb_encoder_deps="libx264"
+libx264rgb_encoder_select="libx264_encoder"
 libx265_encoder_deps="libx265"
 libxavs_encoder_deps="libxavs"
 libxvid_encoder_deps="libxvid"