use gvdb as a subproject

this fixes the build when using --wrap-mode=nofallback.

fix proposed by @eschwartz in gvdb#3 (comment 1484668)

Without this fix, trying to build glib with --wrap-mode=nofallback fails with

Run-time dependency gvdb found: NO (tried pkgconfig and cmake)
Not looking for a fallback subproject for the dependency gvdb because:
Use of fallback dependencies is disabled.
source_subfolder/meson.build:2045:0: ERROR: Dependency 'gvdb' is required but not found.