Let releaseme know to skip po install
Since KF5I18n is optional AND dependent on HEAPTRACK_BUILD_GUI, we have to call ki18n_install(po) conditionally on our own and in addition tell releaseme not to touch CMakeLists.txt to do the same.
Otherwise releaseme adds the following code unconditionally at the end of the file:
find_package(KF5I18n CONFIG REQUIRED)
ki18n_install(po)
ki18n_install checks for existence of po subdir and returns silently if none found, so it is safe for git sources too.
See also downstream report: https://bugs.gentoo.org/829606