QtAutogen: Fix rcc rebuild with Ninja generator (#15459)

The rcc outputs are byproducts of the cmake_autogen rule, not outputs.
We still must run cmake_autogen on every run even if the rcc outputs
exist.  Ninja requires explicit byproduct specification, which is now
possible in CMake since commit v3.2.0-rc1~340^2~2 (Add an option for
explicit BYPRODUCTS of custom commands, 2014-11-13).  Revise the logic
introduced by commit v3.2.0-rc1~480^2 (QtAutogen: Regenerate qrc files
if their input changes, 2014-09-17) to specify byproducts explicitly.

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