Remove 'const' from set_x509_ext()'s value parameter

Remove 'const' from set_x509_ext()'s value parameter

Silences the following warning when compiling with OpenSSL 1.0.2t.

     openssl.c:1213:47: warning: passing 'const char *' to parameter of type 'char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
        ext = X509V3_EXT_conf_nid(NULL, &ctx, nid, value);
                                                   ^~~~~
     /usr/include/openssl/x509v3.h:627:43: note: passing argument to parameter 'value' here
                                         char *value);
                                               ^
Sponsored by: Robert Klemme