Compatibilty with glibc-2.23

In glibc-2.23 <sys/sysmacros.h> isn't automatically included in
<sys/types.h> [1], so we need ot explicitely include it.

https://sourceware.org/ml/libc-alpha/2015-11/msg00253.html

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Justin Lecher <jlec@gentoo.org>
Closes openzfs#6132

lib/libspl/getmntany.c

@@ -34,6 +34,7 @@
 #include <sys/mnttab.h>
 
 #include <sys/types.h>
+#include <sys/sysmacros.h>
 #include <sys/stat.h>
 #include <unistd.h>