Avoid PAGESIZE redefinition

Add #ifndef PAGESIZE to avoid redefinition warning on platforms
where this value is already provided.

Signed-off-by: stf <s@ctrlc.hu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#382

include/sys/sysmacros.h

@@ -102,9 +102,12 @@
 #define PRIO_TO_NICE(prio)		((prio) - MAX_RT_PRIO - 20)
 #endif
 
-/* Missing macros
+/*
+ * Missing macros
  */
+#ifndef PAGESIZE
 #define PAGESIZE			PAGE_SIZE
+#endif
 
 /* from Solaris sys/byteorder.h */
 #define BSWAP_8(x)	((x) & 0xff)