x11/xdg-desktop-portal-wlr: unbreak build with clang 19

../src/screenshot/screenshot.c:129:11: error: variable length array folded to constant array as an extension [-Werror,-Wgnu-folding-constant]

129 |         char uri[strlen(path) + strlen(uri_prefix) + 1];
    |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported by: pkg-fallout