avutil: Add av_get_time_base_q()

This fixes usage of AV_TIME_BASE_Q in C++ applications, which
cannot use compound literals directly in their code.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavutil/utils.c

@@ -113,3 +113,8 @@ unsigned av_int_list_length_for_size(unsigned elsize,
     }
     return i;
 }
+
+AVRational av_get_time_base_q(void)
+{
+    return (AVRational){1, AV_TIME_BASE};
+}