Avoid dependency on uuid.h in MacOSXDYLD
r241964 has added a dependency on uuid.h, which (on linux at least) necessitates instalation of a
new package. Since the only thing we need from that file is uuid_t (and this is already defined
in UuidCompatibility.h), we can avoid this dependency by making this include APPLE specific.
If in future, we need more from this library, we can revisit this decision.
Differential Revision: http://reviews.llvm.org/D11135