Eliminate a shift/reduce conflict.

By simply allowing for the argument_list production to be empty rather
than the lower-level argument production to be empty.
diff --git a/glcpp-parse.y b/glcpp-parse.y
index 27b5514..e70b329 100644
--- a/glcpp-parse.y
+++ b/glcpp-parse.y
@@ -149,7 +149,10 @@
 ;
 
 argument_list:
-	argument {
+	/* empty */ {
+		$$ = _argument_list_create (parser);
+	}
+|	argument {
 		$$ = _argument_list_create (parser);
 		_argument_list_append ($$, $1);
 	}
@@ -164,8 +167,9 @@
 ;
 
 argument:
-	/* empty */ {
+	word {
 		$$ = _string_list_create (parser);
+		_string_list_append_item ($$, $1);
 	}
 |	argument word {
 		_string_list_append_item ($1, $2);