[PATCH] unbreak xhci-trace compilation

When commit 84a99f6fc5d4 (xhci: add traces for debug messages in
xhci_address_device) was backported to the owl tree as part of commit
5b2ea66f15396 (usb: add owl usb driver), the specific CFLAGS needed for
xhci-trace.c were missed, causing build breakage:

  CC      drivers/usb/host/xhci-trace.o
In file included from drivers/usb/host/xhci-trace.h:151:0,
                 from drivers/usb/host/xhci-trace.c:15:
include/trace/define_trace.h:79:43: fatal error: ./xhci-trace.h: No such file or directory

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
---
 drivers/usb/host/Makefile |    3 +++
 1 file changed, 3 insertions(+)

Index: linux-59dbf6a4998e967eb9c6bdcc9b506c0d96acb26b/drivers/usb/host/Makefile
===================================================================
--- linux-59dbf6a4998e967eb9c6bdcc9b506c0d96acb26b.orig/drivers/usb/host/Makefile
+++ linux-59dbf6a4998e967eb9c6bdcc9b506c0d96acb26b/drivers/usb/host/Makefile
@@ -4,6 +4,9 @@
 
 ccflags-$(CONFIG_USB_DEBUG) := -DDEBUG
 
+# tell define_trace.h where to find the xhci trace header
+CFLAGS_xhci-trace.o := -I$(src)
+
 isp1760-y := isp1760-hcd.o isp1760-if.o
 
 fhci-y := fhci-hcd.o fhci-hub.o fhci-q.o