util/thread_local.h: silence a clang-build warning

util/thread_local.h: silence a clang-build warning

otherwise clang complains with

/home/jenkins/workspace/ceph-master/src/rocksdb/util/thread_local.h:205:5:
error: macro expansion producing 'defined' has undefined behavior
[-Werror,-Wexpansion-to-defined]
^
/home/jenkins/workspace/ceph-master/src/rocksdb/util/thread_local.h:22:4:
note: expanded from macro 'ROCKSDB_SUPPORT_THREAD_LOCAL'
!defined(OS_WIN) && !defined(OS_MACOSX) && !defined(IOS_CROSS_COMPILE)
^`

Signed-off-by: Kefu Chai <tchaikov@gmail.com>