qt module: fix missing sanity check for install_dir + install
qt module: fix missing sanity check for install_dir + install

Regression in commit 11f96380351a88059ec55f1070fdebc1b1033117. When
`install: true` the install_dir must be set. The CustomTarget
initializer no longer checks this for us, so we must include the check
ourselves.

pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson