pkgconfig: Make external deps of static library public
pkgconfig: Make external deps of static library public

This fix a regression caused by
https://github.com/mesonbuild/meson/pull/7837, it wanted to make
InternalDependency's external_deps private but has side effect of making
StatisLibrary's external_deps private too.

It is technically correct to make them private, but Meson used to make
StaticLibrary deps public so they are usable without `pkg-config
--static` when we know there is only a static library available.

Fixes: #7929.

jpakkane/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,pexip/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,QuLogic/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,QuLogic/meson,pexip/meson,jpakkane/meson,jpakkane/meson,pexip/meson