lib: bump SONAME

lib: bump SONAME

Based on discussions with amdragon, tschwinge, and others on IRC, I concluded that

1) symbol versioning was probably overkill for libnotmuch
2) It was also probably GNU ld specific
3) Most importantly, nobody could tell me on short notice how exactly it works.

So since the change to the notmuch_database_find_message breaks the
previous ABI, we need to bump the SONAME.