dependencies: add sysroot path to Qt include and library paths

Meson probes the build-host filesystem for Qt private headers when
resolving a Qt dependency during a cross-build. This can cause build-host
include paths (e.g. /usr/include/6.11.0 and Qt subdirs) to be injected
into the cross-compiler command line, which breaks strict cross-compilation
(GCC errors with "unsafe for cross-compilation").

This actually happens only for freedesktop.org pkg-config, because pkgconf
instead adds the sysroot automatically.  Fix by doing it by hand in qt.py.

Fixes: #15456

mesonbuild/dependencies/qt.py

diff --git a/mesonbuild/dependencies/qt.py b/mesonbuild/dependencies/qt.py
index 0dc5d52..a235471 100644
--- a/mesonbuild/dependencies/qt.py
+++ b/mesonbuild/dependencies/qt.py
@@ -193,7 +193,9 @@
                 self.is_found = False
                 return
             if self.private_headers:
+                # fd.o pkg-config does not apply PKG_CONFIG_SYSROOT_DIR to variables.
                 qt_inc_dir = mod.get_variable(pkgconfig='includedir')
+                qt_inc_dir = self.env.get_sys_root_path(self.for_machine, qt_inc_dir)
                 mod_private_dir = os.path.join(qt_inc_dir, 'Qt' + m)
                 if not os.path.isdir(mod_private_dir):
                     if self.env.machines[self.for_machine].is_darwin():
@@ -203,6 +205,7 @@
                         # headers; both public and private headers live under
                         # <libdir>/Qt<Module>.framework/Headers.
                         libdir = mod.get_variable(pkgconfig='libdir', default_value='')
+                        libdir = self.env.get_sys_root_path(self.for_machine, libdir)
                         framework_inc = os.path.join(libdir, f'Qt{m}.framework', 'Headers')
                         if libdir and os.path.isdir(framework_inc):
                             mod_private_dir = framework_inc