And fix build_struct for built-in struct types.

The previous fix broke builtin structs (eg, @param). Calling find_type()
for built-in structs has proven to be a bad idea :)

tools/qfcc/source/struct.c

@@ -129,6 +129,7 @@ build_struct (int su, symbol_t *tag, symtab_t *symtab, type_t *type)
 				symtab->size = size;
 		}
 	}
+	if (!type)
 		sym->type = find_type (sym->type);	// checks the tag, not the symtab
 	sym->type->t.symtab = symtab;
 	return sym;