Fix build issue on some configured kernels

The SPL fails to build with some "Configured" kernels (ex. openSUSE
xen Kernel) this change should make same binaries with C compiler
optimization.

Signed-off-by: zgock <zgock@nuc.base.zgock-lab.net>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#510

include/sys/time.h

@@ -46,7 +46,7 @@
 #define	MSEC2NSEC(m)	((hrtime_t)(m) * (NANOSEC / MILLISEC))
 #define	NSEC2MSEC(n)	((n) / (NANOSEC / MILLISEC))
 
-#define	hz				HZ
+static const int hz = HZ;
 
 #define	TIMESPEC_OVERFLOW(ts)		\
 	((ts)->tv_sec < TIME_MIN || (ts)->tv_sec > TIME_MAX)