gnome: don't let fortify defines into the g-ir-scanner after stripping -O
gnome: don't let fortify defines into the g-ir-scanner after stripping -O

The tool needs to run the preprocessor (but does not actually produce
compiled outputs), and meanwhile ignores lots of flags it doesn't think
it needs. In the case of -D_FORTIFY_SOURCE=...  this is only valid if -O
is there too, but otherwise spits out confusing warnings.

The warnings are spurious and can be safely ignored, but in this case
let's go the extra mile and fix g-ir-scanner's upstream bug by removing
the fortify flag first.

Fixes #9161

pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson