avformat/rtsp: Fix build failure when RTP demuxers are disabled

avformat/rtsp: Fix build failure when RTP demuxers are disabled

rtsp.c uses a check of the form "if (CONFIG_RTSP_DEMUXER && ...) {}"
with the intent to make the code compilable even though the part guarded
by this check contains calls to functions that don't exist when the RTSP
demuxer is disabled. Yet even then compilers still need a declaration of
all the functions in the dead code block and error out if not (due to
our usage of -Werror=implicit-function-declaration) and no such
declaration exists for a static function in rtsp.c. Simply adding a
declaration leads to a "used but never defined" warning, therefore this
commit resorts to an #if.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>