MAINTAINERS: Add usb.h entry to usb

Add usb.h file into the USB list.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 92dda40..9c1543f 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -937,6 +937,7 @@
 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>