build: Update generated sdbus++

The version was bumped recently and we want to use the latest versions.

Change-Id: Ie174703705b63ec780d3b6b7ed11fa03bb3c7545
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/gen/meson.build b/gen/meson.build
index af850f2..56193c7 100644
--- a/gen/meson.build
+++ b/gen/meson.build
@@ -4,10 +4,10 @@
     '--version',
 ).stdout().strip().split('\n')[0]
 
-if sdbuspp_gen_meson_ver != 'sdbus++-gen-meson version 2'
+if sdbuspp_gen_meson_ver != 'sdbus++-gen-meson version 3'
     warning('Generated meson files from wrong version of sdbus++-gen-meson.')
     warning(
-        'Expected "sdbus++-gen-meson version 2", got:',
+        'Expected "sdbus++-gen-meson version 3", got:',
         sdbuspp_gen_meson_ver
     )
 endif