Keep Makefile.am from double-installing detail/Clock.h

Keep Makefile.am from double-installing detail/Clock.h

Summary: On non-Linux systems, the autotools-based build system conditionally appends `detail/Clock.h` to `libfollybase_la_SOURCES` – even though that file is in there statically no matter what (as per [line 46](https://github.com/facebook/folly/blob/master/folly/Makefile.am#L46)), which causes the `install` command to error out trying to install the file twice, during `make install`… this patch takes out the conditional append.
Closes #201

Reviewed By: @yfeldblum

Differential Revision: D2148004

Pulled By: @sgolemon