Add missing detail/Futex-inl.h header (#926)

Summary:
4d234b99 introduces `folly/detail/Futex-inl.h`. This patch adds it to `Makefile.am` so it gets installed when building using autotools.
Pull Request resolved: https://github.com/facebook/folly/pull/926

Reviewed By: yfeldblum

Differential Revision: D9757810

Pulled By: Orvid

fbshipit-source-id: 42bdaf292d9ca5e47c01d39381c5ebd4941dcb2a

folly/Makefile.am

@@ -91,6 +91,7 @@ nobase_follyinclude_HEADERS = \
 	detail/FileUtilDetail.h \
 	detail/FingerprintPolynomial.h \
 	detail/Futex.h \
+	detail/Futex-inl.h \
 	detail/GroupVarintDetail.h \
 	detail/IPAddress.h \
 	detail/IPAddressSource.h \