libzpool: set_global_var: fix endianness handling (fixes zdb -o )

Without this patch I get the error

  Setting global variables is only supported on little-endian systems

when using `zdb -o` on my amd64 machine.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Signed-off-by: Christian Schwarz <me@cschwarz.com>
Closes #11602

lib/libzpool/util.c

@@ -159,7 +159,7 @@ set_global_var(char *arg)
 	char *varname = arg, *varval;
 	u_longlong_t val;
 
-#ifndef _LITTLE_ENDIAN
+#ifndef _ZFS_LITTLE_ENDIAN
 	/*
 	 * On big endian systems changing a 64-bit variable would set the high
 	 * 32 bits instead of the low 32 bits, which could cause unexpected