meson: Use internal not-found dependency for dependencies

Otherwise when other projects such as gstreamer try to use giounix_dep
on Windows with a version check, meson errors out.

This is a Meson bug, but this is more correct since the type of the
dependency is now the same in all cases and it will take a while for
the fix to propagate anyway.

See: https://github.com/mesonbuild/meson/pull/3987
diff --git a/gio/meson.build b/gio/meson.build
index 4b2c8f2..5987a3f 100644
--- a/gio/meson.build
+++ b/gio/meson.build
@@ -845,11 +845,10 @@
   include_directories : [configinc, gioinc])
 
 if host_system == 'windows'
-  # Hack till https://github.com/mesonbuild/meson/issues/2324 is fixed
-  libgiounix_dep = dependency('', required : false)
+  libgiounix_dep = declare_dependency()
   libgiowin32_dep = libgio_dep
 else
-  libgiowin32_dep = dependency('', required : false)
+  libgiowin32_dep = declare_dependency()
   libgiounix_dep = libgio_dep
 endif