pkg-config does not expose libexecdir for finding some tools

On Qt6 6.1+, moc, rcc and uic is placed on INSTALL_LIBEXECDIR on CMake, which can set by builder as he want. When we will try to locate Qt binaries with Qt build such way by pkg-config, we does not have a relevant way to do this, so, cannot find them by relevant way.

I think we should expose INSTALL_LIBEXECDIR as libexecdir in .pc file to solve this issue, and then we will find moc by simply appending executable name to libexecdir (as we can do with qmake now).

I attached a simple patch to fix this issue, but cannot push it directly due to inability to do ssh behind a firewall.