Don't generate subdir convenience rules for IMPORTED targets.

This was missing from commit c34968a9 (Port some of the generator
API to cmGeneratorTarget., 2012-10-10). The generator targets
stored with the cmMakefile include IMPORTED targets, unlike the
accessor for resgular targets. Before this patch, rules would
be generated for Qt5::Core for example, which result in broken
makefiles.
Merge topic 'use-generator-target' into next

02a545c Don't generate subdir convenience rules for IMPORTED targets.