prepare-release: ignore newer version symbols

The script takes the version from the changelog, but if it lacks behind
and the symbols file already includes symbols tagged for the next
version the helper prints incorrect lines as NEW for these symbols, but
ideally it shouldn't print them at all as the symbol is already dealt
with.

Git-Dch: Ignore

prepare-release

@@ -21,7 +21,7 @@ librarysymbolsfromfile() {
 	echo '=== Missing required symbols:'
 	echo -n "$MISSING" | grep -v '|optional=' || true
 	echo '=== New symbols:'
-	grep '^+ ' "$1" | cut -d' ' -f 2 | cut -d'@' -f 1 | c++filt | while read line; do
+	grep '^+ ' "$1" | grep -v '^+ (c++' | cut -d' ' -f 2 | cut -d'@' -f 1 | c++filt | while read line; do
 		echo " (c++)\"${line}@${SYMVER}\" $VERSION"
 	done | sort -u
 }