tools/util-linux: include macOS system uuid header

tools/util-linux: include macOS system uuid header

The type definition of uuid_string_t and possibly other details
used by macOS SDKs like XCode is missing from util-linux.

Headers on macOS have a different inclusion guard
compared to the unique util-linux "_UL" prefix.

This uuid.h header is guaranteed to be present since macOS 10.8 and iOS 6
even without the presence of XCode or other SDKs on the system,
so adding an include_next directive is safe after checking for clang.

Link: https://developer.apple.com/documentation/foundation/uuid
Link: https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/uuid.3.html
Tested-by: Georgi Valkov <gvalkov@gmail.com> # macOS
Signed-off-by: Michael Pratt <mcpratt@pm.me>
Link: https://github.com/openwrt/openwrt/pull/16522
Signed-off-by: Robert Marko <robimarko@gmail.com>