libspl: fix warning about missing spl_pagesize declaration

Though it's unlikely anyone will alter its signature, avoids any
possible type mismatch.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Érico Nogueira <erico.erc@gmail.com>
Closes #12567

lib/libspl/page.c

@@ -21,6 +21,7 @@
  */
 
 #include <unistd.h>
+#include <sys/param.h>
 
 static size_t pagesize = 0;