fix compilation with Centos 5.8

wangle/ThreadGate depends at folly/detail/Futex.h and isn't used in hhvm
while folly/detail/Futex.h doesn't compiles with Centos 5.8 - Linux 2.6.18 , because FUTEX_WAIT_BITSET, FUTEX_WAKE_BITSET, FUTEX_CLOCK_REALTIME aren't defined