message: Fix the release build for xtensa.
  Tested the following command now passes:
  ```
  make -f tensorflow/lite/micro/tools/make/Makefile -j8 TARGET=xtensa OPTIMIZED_KERNEL_DIR=xtensa TARGET_ARCH=hifimini XTENSA_CORE=mini1m1m_RG keyword_benchmark BUILD_TYPE=release
  ```
  Fixes
diff: @@ -31,6 +31,10 @@ PLATFORM_FLAGS = \
  -DMAX_RFFT_PWR=9 \
  -DMIN_RFFT_PWR=MAX_RFFT_PWR
  +ifeq ($(BUILD_TYPE), release)
  + PLATFORM_FLAGS += -Wno-unused-private-field
  +endif
  +
  export PATH := $(XTENSA_BASE)/tools/$(XTENSA_TOOLS_VERSION)/XtensaTools/bin:$(PATH)
  TARGET_TOOLCHAIN_PREFIX := xt-
  