Don't allow adb to run as root in user builds.

Change-Id: I4fbd7beb7f76c68fead1a89b745f0dfb931b4597
diff --git a/adb/Android.mk b/adb/Android.mk
index 7744d2b..248208a 100644
--- a/adb/Android.mk
+++ b/adb/Android.mk
@@ -133,6 +133,10 @@
 LOCAL_CFLAGS += -DANDROID_GADGET=1
 endif
 
+ifneq (,$(filter userdebug eng,$(TARGET_BUILD_VARIANT)))
+LOCAL_CFLAGS += -DALLOW_ADBD_ROOT=1
+endif
+
 LOCAL_MODULE := adbd
 
 LOCAL_FORCE_STATIC_EXECUTABLE := true