Export GSS_KRB5_GET_CRED_IMPERSONATOR on Windows

Add the new public data symbol GSS_KRB5_GET_CRED_IMPERSONATOR to the
gssapi DLL export list.

https://github.com/krb5/krb5/commit/79b87d806ad675b740e440a5b5e6074866fb5f53
Author: Greg Hudson <ghudson@mit.edu>
Commit: 79b87d806ad675b740e440a5b5e6074866fb5f53
Branch: master
 src/lib/gssapi32.def |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)