x264enc: fix build with newer x264 with support for multiple bit depths

x264enc: fix build with newer x264 with support for multiple bit depths

libx264 used to be built for one specific bit depth, and if we
wanted to support multiple bit depths we would have to dynamically
load the right .so from different paths. That has changed now, and
libx264 can include support for multiple depths in the same lib,
so we don't need to do the dlopen() dance any more. We'll keep
the vtable stuff around until we can drop support for older x264.

gstx264enc.c:2927:36: error: ‘x264_bit_depth’ undeclared

https://bugzilla.gnome.org/show_bug.cgi?id=792111