Make ffmpeg's version.sh run in the ffmpeg dir

It seems both our code and ffmpeg's code use version.sh, and read VERSION.  The
problem was that that minor difference was missed in merging the ffmpeg makefile
and we were reading MythTV's VERSION inside ffmpeg, creating crappy version.h

Thanks to superm and xris for finding this before the hustle of a release when
VERSION is normally used.
(cherry picked from commit 864e8db)

Signed-off-by: Stuart Morgan <smorgan@mythtv.org>

mythtv/external/FFmpeg/Makefile

@@ -107,7 +107,7 @@ SVN_ENTRIES = $(SRC_PATH_BARE)/.svn/entries
 .version: M=@
 
 version.h .version:
-	$(M)$(VERSION_SH) $(SRC_PATH) version.h $(EXTRA_VERSION)
+	$(M)$(VERSION_SH) $(SRC_PATH)/external/FFmpeg version.h $(EXTRA_VERSION)
 	$(Q)touch .version
 
 # force version.sh to run whenever version might have changed