windows: Avoid -Wint-conversion errors

Clang 15 made "incompatible pointer to integer conversion" an error instead of a plain warning. This fixes errors like these:

system/keys-win.c:257:13: error: incompatible pointer to integer conversion initializing 'HCRYPTHASH' (aka 'unsigned long') with an expression of type 'void *' [-Wint-conversion]
        HCRYPTHASH hHash = NULL;
                   ^       ~~~~

@mstorsjo the change itself looks good to me, though the commit is based on an old revision from 1.5 years ago. Could you rebase it?

added 691 commits

• 2cfd35c2...1ec2ec20 - 690 commits from branch gnutls:master
• 88d79b96 - windows: Avoid -Wint-conversion errors

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enabled an automatic merge when the pipeline for 88d79b96 succeeds

mentioned in commit 5acc719e

merged