Add default condition for xnnpack_aggregate_library in build_def.bzl

- Fixes TensorFlow build on s390x
- Remove internal gcc -Wgnu-empty-struct which is not supported on GCC version 13

Fixes #6359

PiperOrigin-RevId: 631870251

build_defs.bzl

@@ -270,6 +270,7 @@ def xnnpack_aggregate_library(
             "//build_config:emscripten_wasmsimd": wasmsimd_deps,
             "//build_config:emscripten_wasmrelaxedsimd": wasmrelaxedsimd_deps,
             "//build_config:riscv": riscv_deps,
+            "//conditions:default": [],
         }),
         defines = defines,
         compatible_with = compatible_with,