ui-shared: fix return type of cgit_self_link

cgit_self_link() is a void function but implements each case it handles
by doing "return <another_void_function>" which is not valid C; section
6.8.6.4 of C11 says:

	A return statement with an expression shall not appear in a
	function whose return type is void.

Fix this by removing the return keywords and converting the final code
block into an "else".

Signed-off-by: John Keeping <john@keeping.me.uk>