Define _GNU_SOURCE in hh_shared.c
Define _GNU_SOURCE in hh_shared.c

Summary:
We use some glibc-isms in this file, like testing for MFD_CLOEXEC. Other
toolchains (musl) support this, but only when _GNU_SOURCE is defined.

Reviewed By: mroch

Differential Revision: D30021715

fbshipit-source-id: 4493e1ae876b44c5ad8d04b1ce25ad4019b9620f

mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow