lavc: Move start code finding to utils.c

This allows dropping the mpegvideo dependency from a number of
components.

This also fixes standalone building of the h264 parser, which
was broken in 64e438697.

Signed-off-by: Martin Storsjö <martin@martin.st>
lavc: Rename avpriv_mpv_find_start_code after moving out from mpegvideo

Also move the declaration to internal.h, and add restrict qualifiers
to the declaration (as in the implementation).

Signed-off-by: Martin Storsjö <martin@martin.st>