lib: bump libnotmuch minor version

Notmuch 0.32 corresponds to libnotmuch 5.4 as indicated by docstrings;
however, the minor number wasn't bumped. Any libnotmuch downstream
consumer using the LIBNOTMUCH_CHECK_VERSION macro to support multiple
versions won't be able to access the new 5.4 functions.

Signed-off-by: Austin Ray <austin@austinray.io>