gskit: make sure to terminate version string

In case a very small buffer was passed to the version function, it could
result in the buffer not being NULL-terminated since strncpy() doesn't
guarantee a terminator on an overflowed buffer. Rather than adding code
to terminate (and handle zero-sized buffers), move to using snprintf()
instead like all the other vtls backends.

Closes #3105
Reviewed-by: Daniel Stenberg <daniel@haxx.se>
Reviewed-by: Viktor Szakats <commit@vszakats.net>