avformat/rtsp: Put strings instead of pointers to strings into array

avformat/rtsp: Put strings instead of pointers to strings into array

In this example, the difference in length between the shortest and
longest string is three, so that not using pointers to strings saves
space even on 32bit systems.

Moreover, there is no need to use a sentinel here; it can be replaced
with FF_ARRAY_ELEMS.

Reviewed-by: Ross Nicholson <phunkyfish@gmail.com>
Reviewed-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>