avcodec/version: re-enable FF_API_MPV_OPT until the open ABI period is over

ffserver depends on it, so keep it in place until the two can be
removed together.

libavcodec/version.h

@@ -161,7 +161,7 @@
 #define FF_API_AVCTX_TIMEBASE    (LIBAVCODEC_VERSION_MAJOR < 59)
 #endif
 #ifndef FF_API_MPV_OPT
-#define FF_API_MPV_OPT           (LIBAVCODEC_VERSION_MAJOR < 58)
+#define FF_API_MPV_OPT           (LIBAVCODEC_VERSION_MAJOR < 59)
 #endif
 #ifndef FF_API_STREAM_CODEC_TAG
 #define FF_API_STREAM_CODEC_TAG  (LIBAVCODEC_VERSION_MAJOR < 58)