qt6-qtmultimedia: prevent Xcode 15 build failure

[chrstphrchvz]

See: https://trac.macports.org/ticket/68276#comment:7

Description

Type(s)

• bugfix

Tested on

macOS 13.6 Intel
Command Line Tools 15

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL?
• checked your Portfile with port lint --nitpick?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@MarcusCalhoun-Lopez for port qt6.