Fix oss build

Summary: Fixing oss build

Test Plan: Visual analysis.

Reviewed By: pavlo@fb.com

Subscribers: folly-diffs@, yfeldblum, chalfant

FB internal diff: D2023889

Signature: t1:2023889:1430144784:e58fe9e1f1193d71527ba5f5f2c2efb60bce7a2d

Blame Revision: D2022859

folly/Makefile.am

@@ -135,6 +135,7 @@ nobase_follyinclude_HEADERS = \
 	futures/Timekeeper.h \
 	futures/Try-inl.h \
 	futures/Try.h \
+	futures/Unit.h \
 	futures/detail/Core.h \
 	futures/detail/FSM.h \
 	futures/detail/ThreadWheelTimekeeper.h \