generators: Remove dependency on protos strutil.h

The strutil.h header is not part of the protobuf API
and is removed from later version of protobuf.
Use base::StringUtils instead. Change is reasonably
safe as it affects only tools used at build-time
(the protobuf generators).

Bug: 140126865
Test: check 0-diff in out/xxx/gen/
Change-Id: Iacd1340183e92a6d10f0d033de16bc75ed39013d
diff --git a/src/protozero/protoc_plugin/BUILD.gn b/src/protozero/protoc_plugin/BUILD.gn
index 9cec6b8..432abd8 100644
--- a/src/protozero/protoc_plugin/BUILD.gn
+++ b/src/protozero/protoc_plugin/BUILD.gn
@@ -21,5 +21,6 @@
   deps = [
     "../../../gn:default_deps",
     "../../../gn:protoc_lib",
+    "../../../src/base",
   ]
 }