Eliminate a reduce/reduce conflict in the function-like macro production.

Previously, an empty argument could be parsed as either an "argument_list"
directly or first as an "argument" and then an "argument_list".

We fix this by removing the possibility of an empty "argument_list"
directly.
diff --git a/glcpp-parse.y b/glcpp-parse.y
index 2e40db5..66725db 100644
--- a/glcpp-parse.y
+++ b/glcpp-parse.y
@@ -121,10 +121,7 @@
 ;
 
 argument_list:
-	/* empty */ {
-		$$ = _list_create (parser);
-	}
-|	argument {
+	argument {
 		$$ = _list_create (parser);
 		_list_append_list ($$, $1);
 	}