MAINTAINERS: Add usb.h entry to usb

Add usb.h file into the USB list.
Signed-off-by: NMarek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>

MAINTAINERS

@@ -937,6 +937,7 @@ T:	git https://gitlab.denx.de/u-boot/custodians/u-boot-usb.git
 F:	drivers/usb/
 F:	common/usb.c
 F:	common/usb_kbd.c
+F:	include/usb.h
 
 USB xHCI
 M:	Bin Meng <bmeng.cn@gmail.com>