[grpc] Disable -Wstring-concatenation

https://reviews.llvm.org/rGb2c9b631bb48 moves this warning to -Wall and
we're seeing this triggerred on the clang canary builders.

Bug: 58167
Change-Id: Ibff7cbca873114b7b57e1556c0b7eb84714d2d88
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/grpc/+/416316
Reviewed-by: Roland McGrath <mcgrathr@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index c736bfb..be1bd08e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -24,6 +24,7 @@
   cflags = [
     "-Wno-implicit-int-float-conversion",
     "-Wno-unused-variable",
+    "-Wno-string-concatenation",
   ]
 }