meson_vapi_link.py: Use python's own utils instead of calling 'ln'
meson_vapi_link.py: Use python's own utils instead of calling 'ln'

Replace the use of 'ln' with os.symlink. Since subprocess.call does not
raise any exception when the called process fails, this script likely
silently did nothing on Windows. Instead, fall back to using shutil.copy
to copy instead of symlinking.

GNOME/libgit2-glib,GNOME/libgit2-glib