Avoid PAGESIZE redefinition

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

Signed-off-by: Alec Salazar <alec.j.salazar@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2588

lib/libspl/include/sys/param.h

@@ -57,6 +57,8 @@
 #define	MAXUID		UINT32_MAX	/* max user id */
 #define	MAXPROJID	MAXUID		/* max project id */
 
+#ifndef	PAGESIZE
 #define	PAGESIZE	(sysconf(_SC_PAGESIZE))
+#endif /* PAGESIZE */
 
 #endif