usb: mtu3: mtu3_debug: Add forward declaration of 'struct ssusb_mtk'

Without it, the build system complains that it was declared inside
the parameter list.

Fixes the following W=1 kernel build warning(s):

 In file included from drivers/usb/mtu3/mtu3_trace.c:11:
 drivers/usb/mtu3/mtu3_debug.h:29:36: warning: ‘struct ssusb_mtk’ declared inside parameter list will not be visible outside of this definition or declaration
 29 | void ssusb_dev_debugfs_init(struct ssusb_mtk *ssusb);
 | ^~~~~~~~~
 drivers/usb/mtu3/mtu3_debug.h:30:35: warning: ‘struct ssusb_mtk’ declared inside parameter list will not be visible outside of this definition or declaration
 30 | void ssusb_dr_debugfs_init(struct ssusb_mtk *ssusb);
 | ^~~~~~~~~
 drivers/usb/mtu3/mtu3_debug.h:31:39: warning: ‘struct ssusb_mtk’ declared inside parameter list will not be visible outside of this definition or declaration
 31 | void ssusb_debugfs_create_root(struct ssusb_mtk *ssusb);
 | ^~~~~~~~~
 drivers/usb/mtu3/mtu3_debug.h:32:39: warning: ‘struct ssusb_mtk’ declared inside parameter list will not be visible outside of this definition or declaration
 32 | void ssusb_debugfs_remove_root(struct ssusb_mtk *ssusb);
 | ^~~~~~~~~

Cc: Chunfeng Yun <chunfeng.yun@mediatek.com>
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200703174148.2749969-2-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/mtu3/mtu3_debug.h

@@ -12,6 +12,8 @@
 
 #include <linux/debugfs.h>
 
+struct ssusb_mtk;
+
 #define MTU3_DEBUGFS_NAME_LEN 32
 
 struct mtu3_regset {