Automoc: Fix automoc for OBJECT libraries.

Before this patch,
add_library(obj OBJECT helper.cpp)
add_executable(tgt $<TARGET_OBJECTS:obj>)
didn't run automoc on helper.cpp.
Automoc: add OBJECT library to QtAutomoc test
Merge topic 'automoc-object' into next

0e35cac Automoc: add OBJECT library to QtAutomoc test
cf3faac Automoc: Fix automoc for OBJECT libraries.