libwebp,DEPS: add 'src' to include_rules

this is in preparation for the libwebp-0.6.1 update; include paths were
changed from '../...' to 'src/...'.

BUG=788275
NOPRESUBMIT=true (due to presubmit confusion about local DEPS)

Change-Id: I608f0271d9132aae9f9681d9514dd61c951ee044
Reviewed-on: https://chromium-review.googlesource.com/792251Reviewed-by: Tom Finegan <tomfinegan@chromium.org>
Commit-Queue: James Zern <jzern@google.com>
Cr-Commit-Position: refs/heads/master@{#519525}

third_party/libwebp/DEPS

@@ -4,4 +4,5 @@ include_rules = [
   '+../enc',
   '+../utils',
   '+../webp',
+  '+src',
 ]