Audio: silence MSVC narrowing warnings in qaudiosystem_p.h


Add explicit casts where size_t buffer sizes narrow to 32-bit
frame counts (C4267). No behavior change.

Pick-to: 6.12
Change-Id: I375a3a8f5e892f9c7929bb45291504d7770f34ed
Reviewed-by: Nils Petter Skålerud <nils.petter.skalerud@qt.io>
commitc8a44a0468d88fc68dbdc4fbeaa4e63303767ad0
parentd4290004baa42dce85082e5fbe21f8af9ee9944e
authorTim Blechmann <tim.blechmann@qt.io>
committerTim Blechmann <tim.blechmann@qt.io>
commit timeThu, 15 Oct 2026 09:19:23 +0000