contrib: gnutls: fix CertEnumCRLsInStore in UWP builds

There was a ming32-only check to use LoadLibrary for this call. It was not
supposed to be done on any mingw64 builds. But the define used was only defined
on mingw64 for 64-bits builds.

Fixes LibVLCSharp#295



Patch from upstream master.

(cherry picked from commit 540a7a14)

Signed-off-by: Steve Lhomme <robux4@ycbcr.xyz>