invert Linux handling
invert Linux handling

Rather than checking for non-Linux and including the target specific
header, invert the check and include the linux target header if
`__linux__` is defined.  This makes it easier to target other platforms.

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