backends/ninja: only pass project specific arguments to scan-build

Currently all arguments are being passed to scan-build as part of the
refactoring of how Meson internally handles arguments, but that's wrong,
only project specific arguments are supposed to be passed.

Fixes: #8818

mesonbuild/backend/ninjabackend.py

@@ -3103,6 +3103,7 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
     def get_user_option_args(self):
         cmds = []
         for (k, v) in self.environment.coredata.options.items():
+            if k.is_project():
                 cmds.append('-D' + str(k) + '=' + (v.value if isinstance(v.value, str) else str(v.value).lower()))
         # The order of these arguments must be the same between runs of Meson
         # to ensure reproducible output. The order we pass them shouldn't