Synchronize forward declaration with definition

The actual definition uses "class", so the forward declarations should do
so, too.

include/class_title_block.h

@@ -28,7 +28,7 @@
 
 
 class OUTPUTFORMATTER;
-struct IO_ERROR;
+class IO_ERROR;
 
 
 /**

include/wxPcbStruct.h

@@ -63,7 +63,7 @@ class PCB_LAYER_BOX_SELECTOR;
 class NETLIST;
 class REPORTER;
 struct PARSE_ERROR;
-struct IO_ERROR;
+class IO_ERROR;
 class FP_LIB_TABLE;
 
 namespace PCB { struct IFACE; }     // KIFACE_I is in pcbnew.cpp