Remove unused include of sys/user.h
Remove unused include of sys/user.h

The include of sys/user.h is not needed; it
is a leftover from the initial linux port.
Eliminating it avoids a build problem for
the Swift dispatch overlay on ubuntu 14.04
powerpc64le because the content of sys/user.h
confuses the clang module importer when it
attempts to import CDispatch.

Signed-off-by: Daniel A. Steffen <bc823475b60dbcbd7be1bbdfe095b5f1939d5bd2@apple.com>

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch