mmdevapi: Import get_device_guid() from driver.

dlls/mmdevapi/main.c

@@ -98,6 +98,7 @@ static BOOL load_driver(const WCHAR *name, DriverFuncs *driver)
 
 #define LDFC(n) do { driver->p##n = (void*)GetProcAddress(driver->module, #n);\
         if(!driver->p##n) { goto fail; } } while(0)
+    LDFC(get_device_guid);
     LDFC(get_device_name_from_guid);
     LDFC(GetEndpointIDs);
 #undef LDFC

dlls/mmdevapi/mmdevapi_private.h

@@ -42,6 +42,7 @@ typedef struct _DriverFuncs {
      * valid. See enum _DriverPriority. */
     int priority;
 
+    void (WINAPI *pget_device_guid)(EDataFlow flow, const char *name, GUID *guid);
     BOOL (WINAPI *pget_device_name_from_guid)(GUID *guid, char **name, EDataFlow *flow);
     /* ids gets an array of human-friendly endpoint names
      * keys gets an array of driver-specific stuff that is used