qt511.qtdeclarative: fix CMake `qmlcachegen` path

This package contains several CMake files used for setting up its
provided tools for use in other projects build with CMake.
While packaging *ktouch* I found out that the ${_qt5Core_install_prefix}
variable doesn't expand at all, rendering the path to the `qmlcachegen`
binary useless. As a fix, the command itself is used instead of the path
to the binary.