meson: Properly depend on the generated headers

This fixes dependency problems with the generated headers such as

    https://arm01.puri.sm/job/debs/job/deb-libhandy-buster-armhf/263/console

See

    http://mesonbuild.com/Wrap-best-practices-and-tips.html#eclare-generated-headers-explicitly

src/meson.build

@@ -113,7 +113,7 @@ libhandy = shared_library(
 )
 
 libhandy_dep = declare_dependency(
-              sources: libhandy_public_headers,
+              sources: libhandy_generated_headers,
          dependencies: libhandy_deps,
             link_with: libhandy,
   include_directories: include_directories('.'),