Fix dependencies of net-im/nheko

• because mtxclient was listed as a build dependency only (I thought nheko would pick up the .a and statically link it), it would be installed with nheko, but marked "automatic" and unused.
• "pkg autoremove" would remove mtxclient, and then nheko wouldn't start because the mtxclient .so was missing.

Make mtxclient a library dependency instead.