Revert "lib: gpt: Correct variable name used"

This reverts commit bc9522f.
GPT_LOG_LEVEL is only used within the CMakeLists.txt included in the gpt
library folder, which is not included when building the unittests. The
tfm_log variable LOG_LEVEL was set to GPT_LOG_LEVEL in that file, so the
unit tests should set LOG_LEVEL directly.

Change-Id: Ic3260c668897ea4858d9b6f4df50b980cde1e304
Signed-off-by: Frazer Carsley <frazer.carsley@arm.com>

Author: frazer-arm

lib/gpt/unittests/gpt/utcfg.cmake

@@ -27,6 +27,6 @@ list(APPEND MOCK_HEADERS ${TFM_ROOT_DIR}/lib/tfm_vprintf/inc/tfm_vprintf.h)
 list(APPEND MOCK_HEADERS ${TFM_ROOT_DIR}/lib/ext/efi_soft_crc/inc/efi_soft_crc.h)
 
 # Compile-time definitions
-list(APPEND UNIT_TEST_COMPILE_DEFS GPT_LOG_LEVEL=LOG_LEVEL_VERBOSE)
+list(APPEND UNIT_TEST_COMPILE_DEFS LOG_LEVEL=LOG_LEVEL_VERBOSE)
 list(APPEND UNIT_TEST_COMPILE_DEFS TFM_GPT_BLOCK_SIZE=512)