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/avutil.h

@@ -246,6 +246,11 @@ const char *av_get_media_type_string(enum AVMediaType media_type);
 
 #define AV_TIME_BASE_Q          (AVRational){1, AV_TIME_BASE}
 
+/**
+ * Return the fractional representation of the internal time base.
+ */
+AVRational av_get_time_base_q(void);
+
 /**
  * @}
  * @}
@@ -282,6 +287,7 @@ char av_get_picture_type_char(enum AVPictureType pict_type);
 
 #include "common.h"
 #include "error.h"
+#include "rational.h"
 #include "version.h"
 #include "macros.h"
 #include "mathematics.h"