tests: mgmt: mcumgr: Disable clang warning

When building with clang it reports:

tests/subsys/mgmt/mcumgr/settings_mgmt/src/main.c:69:22: error: variable
'test_response_read_data_start' is not needed and will not be emitted
[-Werror,-Wunneeded-internal-declaration]
static const uint8_t test_response_read_data_start[5] = {
                     ^
tests/subsys/mgmt/mcumgr/settings_mgmt/src/main.c:73:22: error: variable
'test_response_read_data_end' is not needed and will not be emitted
[-Werror,-Wunneeded-internal-declaration]
static const uint8_t test_response_read_data_end[1] = {
                     ^

The values of test_response_read_data_start and
test_response_read_data_end are not actually used anywhere in the tests.

Signed-off-by: Tom Hughes <tomhughes@chromium.org>