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
(cherry picked from commit 89056bd7b1)
Link: https://github.com/openwrt/openwrt/pull/23729
Signed-off-by: Robert Marko <robimarko@gmail.com>

tools/util-linux/patches/102-macos-uuid-next.patch

@@ -0,0 +1,13 @@
+--- a/libuuid/src/uuid.h
++++ b/libuuid/src/uuid.h
+@@ -35,6 +35,10 @@
+ #ifndef _UL_LIBUUID_UUID_H
+ #define _UL_LIBUUID_UUID_H
+ 
++#if defined(__clang__) && defined(__APPLE__)
++#include_next <uuid/uuid.h>
++#endif
++
+ #include <sys/types.h>
+ #ifndef _WIN32
+ #include <sys/time.h>