Add support for DFU Runtime class for devices

This is really just a few descriptors and then answering to the
request from the host to reboot into DFU mode.

That latter part is delegated to the app since this is platform
specific.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

src/tusb.h

@@ -87,6 +87,10 @@
   #if CFG_TUD_USBTMC
     #include "class/usbtmc/usbtmc_device.h"
   #endif
+
+  #if CFG_TUD_DFU_RT
+    #include "class/dfu/dfu_rt_device.h"
+  #endif
 #endif