Fix static data to link with -fno-common

-fno-common is the new default in GCC 10, replacing -fcommon in
GCC <= 9, so static data must only be allocated once.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Romain Dolbeau <romain.dolbeau@european-processor-initiative.eu>
Closes #9943

cmd/zpool/zpool_main.c

@@ -76,6 +76,8 @@
 
 #include "statcommon.h"
 
+libzfs_handle_t *g_zfs;
+
 static int zpool_do_create(int, char **);
 static int zpool_do_destroy(int, char **);