Add a weak dependency (Recommends:) on qt5-qtbase-postgresql to core package

This has been something I've meant to do for a while. It's recommended
but not necessary to use the core with a postgresql server-- and the
package supports it, if the qt5 postgresql bindings are installed.
We may as well pull them in by default, but if a user wants to
remove them for some reason, we should allow it. So I think it makes
sense to use a weak dep here.

quassel.spec

@@ -4,7 +4,7 @@
 Name:    quassel
 Summary: A modern distributed IRC system
 Version: 0.13.1
-Release: 1%{?dist}
+Release: 2%{?dist}
 
 License: GPLv2 or GPLv3
 URL:     http://quassel-irc.org/
@@ -76,6 +76,10 @@ Summary: Quassel core component
 # Required for the quassel user.
 Requires(pre): shadow-utils
 
+# Weak dependency on qt5 postgresql bindings.
+# We use a weak dependency here so they can be uninstalled if necessary.
+Recommends: qt5-qtbase-postgresql
+
 %description core
 The Quassel IRC Core maintains a connection with the
 server, and allows for multiple clients to connect
@@ -167,6 +171,9 @@ exit 0
 
 
 %changelog
+* Sat Feb 16 2019 Ben Rosser <rosser.bjr@gmail.com> - 0.13.1-2
+- Add a weak dep (Recommends) on qt5-qtbase-postgresql.
+
 * Fri Feb 15 2019 Ben Rosser <rosser.bjr@gmail.com> - 0.13.1-1
 - Updated to latest upstream release, 0.13.1 (rhbz#1677722).