Fix Clang warning about struct/class mismatch

CaretGeometry is defined as a struct, but was forward-declared as a
class. Now we forward-declare as a struct. This addresses the
following Clang warning (that does not appear with GCC):

src/frontends/CaretGeometry.h:27:1: warning: 'CaretGeometry' defined
as a struct here but previously declared as a class; this is valid,
but may result in linker errors under the Microsoft C++ ABI
[-Wmismatched-tags]

src/BufferView.h

@@ -26,7 +26,7 @@ namespace lyx {
 
 namespace support { class FileName; }
 
-namespace frontend { class CaretGeometry; }
+namespace frontend { struct CaretGeometry; }
 namespace frontend { class Painter; }
 namespace frontend { class GuiBufferViewDelegate; }