Fix build failure due to "multiple definition" errors

The USB descriptor variables were effectively being declared twice--once
in usb_descriptors.c and once in usb_core.c--because both files included
usb_descriptors.h, where those variables were originally declared
without the "extern" keyword. Because of this, GCC refused to link the
object files. After declaring the variables as "extern", the build
finishes without issue.

usb_descriptors.h

@@ -28,7 +28,7 @@ typedef enum {
     usb_string_index_last,
 } __attribute__ ((packed)) usb_string_index_t;
 
-const usb_string_descriptor_t *usb_string_descriptors[usb_string_index_last];
+extern const usb_string_descriptor_t *usb_string_descriptors[usb_string_index_last];
 
 /* Endpoints */
 
@@ -55,7 +55,7 @@ enum {
 
 /* Device Descriptor */
 
-const usb_device_descriptor_t usb_device_descriptor;
+extern const usb_device_descriptor_t usb_device_descriptor;
 
 /* Configuration Descriptor */
 
@@ -93,6 +93,6 @@ typedef struct {
     usb_endpoint_descriptor_t           data_eptx_2;
 } __attribute__((packed)) usb_device_configuration_descriptor_t;
 
-const usb_device_configuration_descriptor_t usb_configuration_descriptor;
+extern const usb_device_configuration_descriptor_t usb_configuration_descriptor;
 
 #endif /* USB_DESCRIPTORS_H */