Build 'client' target with -Wno-sign-compare.

This way we don't get the following warning when building with -Wall:

warning: comparison between signed and unsigned integer expressions
[-Wsign-compare]

--
Change-Id: I259c2398c2d8df5b3444d2d2ed6d45d8c5cfceef
Reviewed-on: https://bazel-review.googlesource.com/1291
MOS_MIGRATED_REVID=93514679
diff --git a/src/main/cpp/BUILD b/src/main/cpp/BUILD
index 3d8b1da..a4aeec9 100644
--- a/src/main/cpp/BUILD
+++ b/src/main/cpp/BUILD
@@ -48,6 +48,7 @@
         ":blaze_util_os",
     ],
     copts = [
+        "-Wno-sign-compare",
         "-DBLAZE_JAVA_CPU=\\\"k8\\\"",
         "-DBLAZE_OPENSOURCE=1",
     ],