No package root default in the build system.

Now that we have '-R', using it in the build system. This makes sure
that if a package root is missing because of programmer error, we
don't accidentally read from that location.

Bug: N/A
Test: build (build only change)
Change-Id: Ie70f131a37dfd0d6d4ec966cf504bb439c10d30d
diff --git a/build/hidl_interface.go b/build/hidl_interface.go
index 82c5c53..3d78223 100644
--- a/build/hidl_interface.go
+++ b/build/hidl_interface.go
@@ -35,7 +35,7 @@
 	hidlRule = pctx.StaticRule("hidlRule", blueprint.RuleParams{
 		Depfile:     "${depfile}",
 		Deps:        blueprint.DepsGCC,
-		Command:     "rm -rf ${genDir} && ${hidl} -p . -d ${depfile} -o ${genDir} -L ${language} ${roots} ${fqName}",
+		Command:     "rm -rf ${genDir} && ${hidl} -R -p . -d ${depfile} -o ${genDir} -L ${language} ${roots} ${fqName}",
 		CommandDeps: []string{"${hidl}"},
 		Description: "HIDL ${language}: ${in} => ${out}",
 	}, "depfile", "fqName", "genDir", "language", "roots")