Belatedly bump version number to 1.12.21 for development

Signed-off-by: Simon McVittie <smcv@collabora.com>

configure.ac

@@ -3,7 +3,7 @@ AC_PREREQ([2.63])
 
 m4_define([dbus_major_version], [1])
 m4_define([dbus_minor_version], [12])
-m4_define([dbus_micro_version], [20])
+m4_define([dbus_micro_version], [21])
 m4_define([dbus_version],
           [dbus_major_version.dbus_minor_version.dbus_micro_version])
 AC_INIT([dbus],[dbus_version],[https://bugs.freedesktop.org/enter_bug.cgi?product=dbus],[dbus])