Forward declare IBlobReader in Volume headers

We have to do this (or include Blob.h) since the
constructors take unique_ptr<IBlobReader> parameters.

Source/Core/DiscIO/VolumeGC.h

@@ -16,6 +16,7 @@
 
 namespace DiscIO
 {
+class IBlobReader;
 enum class BlobType;
 enum class Country;
 enum class Language;

Source/Core/DiscIO/VolumeWiiCrypted.h

@@ -17,6 +17,7 @@
 
 namespace DiscIO
 {
+class IBlobReader;
 enum class BlobType;
 enum class Country;
 enum class Language;