Make ff_random_get_seed public, rename to av_get_random_seed, export the header

Keep an old ff_ named function for binary compatibility until the
next major bump.

Originally committed as revision 23254 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/random_seed.h

@@ -26,6 +26,6 @@
 /**
  * Gets a seed to use in conjunction with random functions.
  */
-uint32_t ff_random_get_seed(void);
+uint32_t av_get_random_seed(void);
 
 #endif /* AVUTIL_RANDOM_SEED_H */