meson: Always build hwdb file

We want systemd to pull our hwdb. In order to ease this, always build
the hwdb file, even if it is disabled.

Once systemd has merged the rules, downstream should turn off the rules
in libfprint. The default in libfprint will also be changed to not build
the hwdb (udev_rules option) eventually.

meson.build

@@ -181,6 +181,8 @@ if get_option('udev_rules')
         udev_dep = dependency('udev')
         udev_hwdb_dir = udev_dep.get_pkgconfig_variable('udevdir') + '/hwdb.d'
     endif
+else
+    udev_hwdb_dir = false
 endif
 
 if get_option('gtk-examples')