Conditionally include linux/udp.h (#2894)

### Motivation:

To enable access to `UDP_MAX_SEGMENTS` we switched from including
`netinet/udp.h` to including `linux/udp.h` however the latter isn't
available in MUSL.

### Modifications:

if `linux/udp.h` is available then import it in favor of
`netinet/udp.h`. If we fall back to the latter then `UDP_MAX_SEGMENTS`
won't be defined but that will be caught by the existing paths which
account for this case.

### Result:

NIO should be able to build against a static MUSL SDK again.

Sources/CNIOLinux/include/CNIOLinux.h

@@ -30,7 +30,11 @@
 #include <errno.h>
 #include <pthread.h>
 #include <netinet/ip.h>
+#if __has_include(<linux/udp.h>)
 #include <linux/udp.h>
+#else
+#include <netinet/udp.h>
+#endif
 #include <linux/vm_sockets.h>
 #include <fcntl.h>
 #include <fts.h>