apps: qcam: Disable -Wsfinae-incomplete

GCC-16 has updated the warnings enabled and now includes
-Wsfinae-incomplete triggering breakage in the included headers from
Qt6:

/usr/include/qt6/QtCore/qregularexpression.h:31:21: error: defining ‘QRegularExpression’, which previously failed to be complete in a SFINAE context [-Werror=sfinae-incomplete=]

This is a Qt header issue outside of libcamera’s control. Disable the
warning for qcam to restore buildability with newer GCC versions.

Link: https://qt-project.atlassian.net/browse/QTBUG-143470
Reviewed-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/apps/qcam/meson.build

@@ -40,6 +40,7 @@ qt6_cpp_args = [
     apps_cpp_args,
     '-DQT_NO_KEYWORDS',
     '-Wno-extra-semi',
+    '-Wno-sfinae-incomplete',
 ]
 
 # gcc 12 and 13 output a false positive variable shadowing warning with Qt