qt4: fix parallel building

qmake needs uic to extract dependency information from .ui files.  In -fast mode
qmake generates Makefiles for each subproject after the previous subproject is
built, and by the time it encounters subprojects with .ui files, uic is
available to provide dependency information.  In -no-fast mode qmake builds
subprojects in the same order, but generates all Makefiles beforehand, without
uic, and they miss dependencies.