adb: Enable secure adb on user/userdebug build

Enable ro.adb.secure property in user/userdebug build

[toddpoynor@google.com: syntax fixup]
Change-Id: I3ecc7598b405da2a603d68c84ee68f2bcdacc341
Signed-off-by: Xerox Lin <xerox_lin@htc.com>
diff --git a/device.mk b/device.mk
index 641e151..8bb98e0 100644
--- a/device.mk
+++ b/device.mk
@@ -172,7 +172,8 @@
 
 # set default USB configuration
 PRODUCT_DEFAULT_PROPERTY_OVERRIDES += \
-    persist.sys.usb.config=mtp
+    persist.sys.usb.config=mtp \
+    ro.adb.secure=1
 
 # for off charging mode
 PRODUCT_PACKAGES += \