libsa: Disable -Wdangling-pointer for zfs.c.

GCC 12 warns about a dangling pointer to 'objid' in
zfs_bootenv_initial().  However, this appears to be a false positive
as the pointer to 'objid' is only passed to zfs_lookup_dataset() but
not saved anywhere that outlives the lifetime of the
zfs_bootenv_initial() function.

Reviewed by:	imp, emaste
Differential Revision:	https://reviews.freebsd.org/D37533

stand/libsa/zfs/Makefile.inc

@@ -83,3 +83,5 @@ CFLAGS.skein_block.c+=	-DSKEIN_LOOP=111
 # To find blake3_impl.c in OpenZFS tree for our somehat ugly blake3_impl_hack.c
 # that's needed until the necessary tweaks can be upstreamed.
 CFLAGS.blake3_impl_hack.c+= -I${OZFS}/module/icp/algs/blake3
+
+CWARNFLAGS.zfs.c+= ${NO_WDANGLING_POINTER}