Fixed compile error when FIXED_CONTROL_ENDPOINT_SIZE compile time option was disabled, and a USE_*_DESCRIPTORS compile time option was not enabled on the AVR8s.

Add C++ compatibility to some header files currently missing extern "C" linkage.

LUFA/Drivers/USB/Core/ConfigDescriptor.h

@@ -256,7 +256,7 @@
 			 */
 			uint8_t USB_GetNextDescriptorComp(uint16_t* const BytesRem,
 			                                  void** const CurrConfigLoc,
-			                                  const ConfigComparatorPtr_t const ComparatorRoutine);
+			                                  ConfigComparatorPtr_t const ComparatorRoutine);
 
 		/* Inline Functions: */
 			/** Skips over the current sub-descriptor inside the configuration descriptor, so that the pointer then