qdoc: Fix compilation warnings on MSVC

We pass the size_t of an std::vector to a clang function taking an
int as an argument, which on MSVC resulted in

warning C4267: 'argument': conversion from 'size_t' to 'int',
               possible loss of data

To fix this, cast explicitly.

Change-Id: I3636f7ced600c387c678aa59012274db490d8f8d
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@qt.io>
Reviewed-by: Martin Smith <martin.smith@qt.io>