LU-14093 gss: gcc10 fixes for GSS

LU-14093 gss: gcc10 fixes for GSS

Building with gcc10 reports the following issues when building
GSS:

gss_util.h:37: multiple definition of `this_realm';
gssd.h:73: multiple definition of `clnt_list';
svcgssd.h:38: multiple definition of `krb_enabled';

Properly scope these variables.

Change-Id: I05fc298fb90d67314c6963273688c2577099188a
Test-Parameters: env=SHARED_KEY=true testlist=sanity,recovery-small,sanity-sec
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/44363
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>