fix compilation for later openssl

[scottjg]

in later openssl, the key param in EVP_PKEY_assign changed from char* to void*. causes this error:

../github.com/spacemonkeygo/openssl/key.go:324: cannot use
(*C.char)(unsafe.Pointer(rsa)) (type *C.char) as type unsafe.Pointer in
argument to _Cfunc_EVP_PKEY_assign

this fix adds a shim function in C, where the char* can be automatically coerced into void*

[thepaul]

+1

[zeebo]

Thanks. :+1: