LibWeb: Include standard SVG user agent style sheet

For now, part of this is commented-out. Our current implementations of
`<mask>` and `<symbol>` rely on creating layout nodes, so they can't be
`display: none`.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -655,6 +655,7 @@ set(GENERATED_SOURCES
     CSS/TransformFunctions.cpp
     CSS/ValueID.cpp
     MathML/MathMLStyleSheetSource.cpp
+    SVG/SVGStyleSheetSource.cpp
 )
 
 serenity_lib(LibWeb web)