Comment # 1 on bug 1172015 from Ralf Habacker

On Linux, the mentioned pc file is provided by the libqt5-qttools-devel package
(for example see
https://build.opensuse.org/package/binary/openSUSE:Leap:15.1/libqt5-qttools/standard/x86_64/libqt5-qttools-devel-5.9.7-lp151.1.38.x86_64.rpm),
which is not available for Mingw. 

Instead this file is located in the mingw32-libqt5-qttools

rpm -q -l  mingw32-libqt5-qttools  | grep \.pc
/usr/i686-w64-mingw32/sys-root/mingw/lib/pkgconfig/Qt5Designer.pc
/usr/i686-w64-mingw32/sys-root/mingw/lib/pkgconfig/Qt5Help.pc
/usr/i686-w64-mingw32/sys-root/mingw/lib/pkgconfig/Qt5UiTools.pc

but this package does not provide the dependency mingw{32,64}(pkg:Qt5Designer)

rpm -q --provides  mingw32-libqt5-qttools | grep pkg
mingw32(pkg:Qt5Help) = 5.9.1
mingw32(pkg:Qt5UiTools) = 5.9.1

Looking at the build log at
https://build.opensuse.org/package/live_build_log/windows:mingw:win32/mingw32-libqt5-qttools/openSUSE_Leap_15.1/x86_64
shows an error on finding 'provides':

[  422s]
~/rpmbuild/BUILDROOT/mingw32-libqt5-qttools-src-5.9.1-lp151.2.43.x86_64
[  422s] Processing files: mingw32-libqt5-qttools-5.9.1-lp151.2.43.noarch
[  422s] warning: Deprecated external dependency generator is used!
[  422s] Finding  Provides: /usr/lib/rpm/mingw32-find-provides.sh
[  422s] Package Qt5UiPlugin was not found in the pkg-config search path.
[  422s] Perhaps you should add the directory containing `Qt5UiPlugin.pc'
[  422s] to the PKG_CONFIG_PATH environment variable
[  422s] Package 'Qt5UiPlugin', required by 'Qt5Designer', not found
...
[  423s] Provides: mingw32(lib:Qt5Designer) mingw32(lib:Qt5DesignerComponents)
mingw32(lib:Qt5Help) mingw32(lib:Qt5UiTools) mingw32(libqquickwidget.dll)
mingw32(libqt5designer.dll) mingw32(libqt5designercomponents.dll)
mingw32(libqt5help.dll) mingw32(pkg:Qt5Help) = 5.9.1 mingw32(pkg:Qt5UiTools) =
5.9.1 mingw32-libqt5-qttools = 5.9.1-lp151.2.43

The related linux package (for example
https://build.opensuse.org/package/binary/download/openSUSE:Factory/libqt5-qttools/standard/x86_64/libqt5-qttools-devel-5.14.1-1.5.x86_64.rpm)
provides that library and pkgconfig file, so the issue is that the
mingw32-libqt5-qttools package does not build that library.