apps: qcam: Disable -Wextra-semi

The Qt 6 qkeysequence.h header has an extra semicolon. This causes a
build failure with clang:

/usr/include/qt6/QtGui/qkeysequence.h:139:26: error: extra ';' after member function definition [-Werror,-Wextra-semi]
  139 |     Q_ENUM(SequenceMatch);
      |                          ^

We can't fix the issue, so ignore the warning in qcam.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Milan Zamazal <mzamazal@redhat.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>