gost_pmeth.c: mitigate for constness difference in copy function

The second argument for the pkey_copy function has been constified in
OpenSSL to be 3.0.  This is normally not a problem, but when passing a
pointer to such a function, the compiler does complain.

This change solves the issue by looking at the macros
OPENSSL_VERSION_MAJOR, which is new in OpenSSL to be 3.0, and set
constness based on that.

Fixes #154