toolchain: xpcg: update fx path api usage

toolchain/xpcg/backend/c-mpc/backend.c

@@ -1150,7 +1150,7 @@ static int emit_interface(const struct interface_definition *iface)
 	snprintf(path, sizeof path, "%s.h", iface->if_name);
 	fx_result result = fx_file_open(
 		NULL,
-		FX_RV_PATH(path),
+		FX_CSTR(path),
 		FX_FILE_WRITE_ONLY | FX_FILE_CREATE | FX_FILE_TRUNCATE,
 		&file);
 	if (fx_result_is_error(result)) {

toolchain/xpcg/main.c

@@ -86,11 +86,8 @@ static int xpcg(
 	}
 
 	fx_file *file = NULL;
-	fx_result result = fx_file_open(
-		NULL,
-		FX_RV_PATH(path),
-		FX_FILE_READ_ONLY,
-		&file);
+	fx_result result
+		= fx_file_open(NULL, FX_CSTR(path), FX_FILE_READ_ONLY, &file);
 	if (fx_result_is_error(result)) {
 		fx_throw(result);
 		return -1;