contrib: gnutls: Backport a patch to 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;
                   ^       ~~~~

This backports
https://gitlab.com/gnutls/gnutls/-/commit/88d79b964d88730e316919d6ccd17ca0fe9b3244.