Swift currently fails to build if `dev-libs/libdispatch` is installed
because it globally installs private `libdispatch` headers which get
unexpectedly picked up during the Swift build. Since the headers are not
expected to be present when building Swift on non-Darwin platforms, the
check for the header can be patched out.
Signed-off-by: Itai Ferber <itai@itaiferber.net>
