If the header file containing a function's prototype isn't included by
the sourcefile containing the associated function, the build system
complains of missing prototypes.

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

 drivers/usb/mtu3/mtu3_trace.c:13:6: warning: no previous prototype for ‘mtu3_dbg_trace’ [-Wmissing-prototypes]
 13 | void mtu3_dbg_trace(struct device *dev, const char *fmt, ...)
 | ^~~~~~~~~~~~~~

Cc: Chunfeng Yun <chunfeng.yun@mediatek.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200715093209.3165641-5-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>