Fix build with --enable-ladspa --disable-frei0r.

https://bugs.gentoo.org/show_bug.cgi?id=510516Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -2499,6 +2499,7 @@ hqdn3d_filter_deps="gpl"
 interlace_filter_deps="gpl"
 kerndeint_filter_deps="gpl"
 ladspa_filter_deps="ladspa dlopen"
+ladspa_filter_extralibs='$ldl'
 mcdeint_filter_deps="avcodec gpl"
 movie_filter_deps="avcodec avformat"
 mp_filter_deps="gpl avcodec swscale inline_asm"