jom's up-to-date check is incomplete for inference rule targets

to reproduce:

• build qmake from Qt 4.7 branch
• touch winmakefile.cpp
• run jom

Nothing happens. Jom thinks that all files are up-to-date.

Reason: applyInferenceRules is only called for leaves. The target "winmakefile.obj" has only the qmake_pch.obj as dependent until the inference rule is applied.