Replace the deprecated proto_source_root attribute. (#241)

With strip_proto_prefix .
diff --git a/test/proto/BUILD b/test/proto/BUILD
index bcb51d2..7217e55 100644
--- a/test/proto/BUILD
+++ b/test/proto/BUILD
@@ -3,14 +3,14 @@
 proto_library(
     name = "a_proto",
     srcs = ["a.proto"],
-    proto_source_root = "test/proto",
+    strip_import_prefix = "",
 )
 
 proto_library(
     name = "b_proto",
     srcs = ["b.proto"],
+    strip_import_prefix = "",
     deps = [":a_proto"],
-    proto_source_root = "test/proto",
 )
 
 rust_proto_library(