Fixes for running FreeBSD buildworld on Linux/macOS hosts

Adding an #ifdef __FreeBSD__ to a FreeBSD-specific header may seem odd,
but these headers are used on non-FreeBSD systems during the bootstrap
tools phase.
Originally submitted downstream as https://reviews.freebsd.org/D26193

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
Closes #10863

lib/libspl/include/os/freebsd/sys/stat.h

@@ -28,6 +28,8 @@
 
 #include_next <sys/stat.h>
 
+/* Note: this file can be used on linux/macOS when bootstrapping tools. */
+#if defined(__FreeBSD__)
 #include <sys/mount.h> /* for BLKGETSIZE64 */
 
 #define	stat64	stat
@@ -68,4 +70,5 @@ fstat64_blk(int fd, struct stat64 *st)
 
 	return (0);
 }
+#endif /* defined(__FreeBSD__) */
 #endif /* _LIBSPL_SYS_STAT_H */