version: make ssl_version buffer match for multi_ssl

When running a multi TLS backend build the version string needs more
buffer space. Make the internal ssl_buffer stack buffer match the one
in Curl_multissl_version() to allow for the longer string. For single
TLS backend builds there is no use in extended to buffer. This is a
fallout from #3863 which fixes up the multi_ssl string generation to
avoid a buffer overflow when the buffer is too small.

Closes #3875
Reviewed-by: Daniel Stenberg <daniel@haxx.se>