windows.yml: Install missing doxygen

Symptom was:
> ##[group]Run cmake -B build_lv -S libvisual -GNinja -DCMAKE_BUILD_TYPE=Debug -DBUILD_SHARED_LIBS=OFF \
>   -DENABLE_DOCS=yes -DENABLE_NLS=OFF -DENABLE_TESTS=yes -DCMAKE_INSTALL_PREFIX="${env:LV_INSTALL_PREFIX}"
> [..]
> -- Found OpenGL: opengl32
> CMake Error at C:/msys64/mingw64/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
>   Could NOT find Doxygen (missing: DOXYGEN_EXECUTABLE)
> Call Stack (most recent call first):
>   C:/msys64/mingw64/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:600 (_FPHSA_FAILURE_MESSAGE)
>   C:/msys64/mingw64/share/cmake/Modules/FindDoxygen.cmake:682 (find_package_handle_standard_args)
>   CMakeLists.txt:254 (FIND_PACKAGE)

.github/workflows/windows.yml

@@ -47,6 +47,7 @@ jobs:
           pacman -Sy --needed --ask=20 --noconfirm `
             mingw-w64-x86_64-cmake `
             mingw-w64-x86_64-dlfcn `
+            mingw-w64-x86_64-doxygen `
             mingw-w64-x86_64-gcc `
             mingw-w64-x86_64-gettext `
             mingw-w64-x86_64-gstreamer `