usb: gadget: use $(srctree) instead of $(PWD) for includes

Using $(PWD) breaks builds when make was invoked from outside
of the kernel tree.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/drivers/usb/gadget/function/Makefile b/drivers/usb/gadget/function/Makefile
index edb6dfa..a49fdf7b 100644
--- a/drivers/usb/gadget/function/Makefile
+++ b/drivers/usb/gadget/function/Makefile
@@ -2,8 +2,8 @@
 # USB peripheral controller drivers
 #
 
-ccflags-y			:= -I$(PWD)/drivers/usb/gadget/
-ccflags-y			+= -I$(PWD)/drivers/usb/gadget/udc/
+ccflags-y			:= -I$(srctree)/drivers/usb/gadget/
+ccflags-y			+= -I$(srctree)/drivers/usb/gadget/udc/
 
 # USB Functions
 usb_f_acm-y			:= f_acm.o