Meta: Define HAVE_QT when building WebContent for Qt in the GN build

For some reason, WebContent fails to load simple sites like xkcd.com
without the Qt event loop, even when using RequestServer instead of the
Qt networking stack. The CMake build on Linux has the same issue if we
skip installing the Qt event loop. It's not clear why this is - whether
something depends on the Qt event loop, or if there's a bug in the Unix
event loop implementation.

This is, however, also needed to use the --enable-qt-networking feature.

Meta/gn/secondary/Ladybird/WebContent/BUILD.gn

@@ -72,6 +72,7 @@ executable("WebContent") {
   ]
 
   if (enable_qt) {
+    defines = [ "HAVE_QT" ]
     configs += [ ":WebContent_qt" ]
     sources += [
       "//Ladybird/Qt/EventLoopImplementationQt.cpp",
@@ -83,6 +84,7 @@ executable("WebContent") {
     ]
 
     if (enable_qt_multimedia) {
+      defines += [ "HAVE_QT_MULTIMEDIA" ]
       sources += [
         "//Ladybird/Qt/AudioCodecPluginQt.cpp",
         "//Ladybird/Qt/AudioThread.cpp",