Use size_t for qHash return values

Otherwise I get the following error:
```log
plasma-framework/src/plasma/svg.cpp:39:35: error: non-constant-expression cannot be narrowed from type 'size_t' (aka 'unsigned long') to 'unsigned int' in initializer list [-Wc++11-narrowing]
    std::array<uint, 10> parts = {::qHash(id.width),
                                  ^~~~~~~~~~~~~~~~~
```