Silence unknown pragmas warning in the WebView2 header

Using wxGCC_WARNING_SUPPRESS(unknown-pragmas) around the webview2 header does
not work, so suppress it for the entire webview project.

build/cmake/lib/webview/CMakeLists.txt

@@ -69,6 +69,7 @@ elseif(WXMSW)
         wx_lib_include_directories(wxwebview PRIVATE "${WEBVIEW2_PACKAGE_DIR}/build/native/include")
         if(NOT MSVC)
             wx_lib_include_directories(wxwebview PRIVATE "${wxSOURCE_DIR}/include/wx/msw/wrl")
+            target_compile_options(wxwebview PRIVATE -Wno-unknown-pragmas)
         endif()
 
         if (CMAKE_SIZEOF_VOID_P EQUAL 4)