yaml: move to subdirectory

A pending sdbusplus change (I38b69bc67b6a3d80cda1b508a76e106a50d8ab93)
is going to require YAML to be in a specific subdirectory and not in
the root.  Move here in prep for that change.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I42cd6599f751b34284243080dd213be3254d147c
diff --git a/gen/meson.build b/gen/meson.build
index 56193c7..2756633 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 3'
+if sdbuspp_gen_meson_ver != 'sdbus++-gen-meson version 4'
     warning('Generated meson files from wrong version of sdbus++-gen-meson.')
     warning(
-        'Expected "sdbus++-gen-meson version 3", got:',
+        'Expected "sdbus++-gen-meson version 4", got:',
         sdbuspp_gen_meson_ver
     )
 endif