doc/doxy-wrapper.sh: use ffbuild/version.sh to determine the version

This removes the duplicated version file lookup, which would have
broken with the rename of the VERSION file to FF_VERSION, and makes
the doxygen PROJECT_NUMBER match FFMPEG_VERSION for git builds.

doc/doxy-wrapper.sh

@@ -9,11 +9,7 @@ shift 4
 
 cd ${SRC_DIR}
 
-if [ -e "VERSION" ]; then
-    VERSION=`cat "VERSION"`
-else
-    VERSION=`git describe`
-fi
+VERSION=$(./ffbuild/version.sh .)
 
 $DOXYGEN - <<EOF
 @INCLUDE        = ${DOXYFILE}