qt5: add pkg-config build dependency

When pkg-config is not available at build-time, it is permanently disabled.
This means qmake link_pkgconfig.prf will not work as documented even if
pkg-config is available at run-time:
https://qt-project.org/faq/answer/does_qmake_have_support_for_pkg-config

Add a build-time dependency to the qt5 formula.

Closes #27184.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>