build: release.mk: deal with trailing whitespace-to-comment-delimiter

... that is preserved by design, see:
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/make.html#tag_20_76_13_05

> string1 = [string2]
> [...]
> where string2 is defined as all characters, if any, after the
> <equals-sign>, up to a comment character ( '#' ) or an unescaped
> <newline>. Any <blank> characters immediately before or after the
> <equals-sign> shall be ignored.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>

build-aux/release.mk

@@ -74,7 +74,7 @@ endif
 
 # NOTE: cannot sign multiple files at once
 $(project)-$(version).%.asc: $(project)-$(version).%
-	gpg --default-key "$(gpgsignkey)" \
+	gpg --default-key "$(strip $(gpgsignkey))" \
 		--detach-sign \
 		--armor \
 		$<