tests: logging/log_syst: fix unused var warnings

When CONFIG_ZTEST_ASSERT_VERBOS is disabled, syst_headers_name[]
is not exactly being used by zassert_mem_equal(). Mark it as
__used to avoid compiler warnings.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

Author: dcpleung

tests/subsys/logging/log_syst/src/mock_backend.c

@@ -63,7 +63,8 @@ void validate_msg(const char *type, const char *optional_flags,
 	const char *raw_data_str = "SYS-T RAW DATA: ";
 	const char *output_str = test_output_buf;
 	const char *syst_format_headers[4] = {type, optional_flags, module_id, sub_type};
-	const char *syst_headers_name[4] = {"type", "optional_flags", "module_id", "sub_type"};
+	const char *syst_headers_name[4] __used = {"type", "optional_flags", "module_id",
+						   "sub_type"};
 
 	/* Validate "SYS-T RAW DATA: " prefix in the output_str */
 	zassert_mem_equal(raw_data_str, output_str, strlen(raw_data_str),