build: remove tmp/ on release archives

i overlooked this before. remove it. the directory
happened to be empty when i tested archives, but it's
still not a good thing that we have it. remove it!

Signed-off-by: Leah Rowe <leah@libreboot.org>

build

@@ -95,7 +95,7 @@ build_release()
 	cd "$srcdir" || $err "$vdir: !cd \"$srcdir\""
 	./update trees -f
 	rmgit .
-	x_ rm -Rf cache
+	x_ rm -Rf cache tmp
 	x_ mv src/docs docs
 	) || $err "can't create release files"