Add -Wno-write-strings

This is being turned on in the main repo.

Change-Id: Id6457ecfe09f702d1f31b40e191fde8096e40dfe
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/sbase/+/577005
Reviewed-by: Shai Barack <shayba@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index eb7959f..35fd744 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -111,6 +111,7 @@
     "-Wno-sign-compare",
     "-Wno-sometimes-uninitialized",
     "-Wno-tautological-constant-out-of-range-compare",
+    "-Wno-write-strings",
   ]
 }
 
diff --git a/libutil/BUILD.gn b/libutil/BUILD.gn
index b42d4d1..f2312fa 100644
--- a/libutil/BUILD.gn
+++ b/libutil/BUILD.gn
@@ -9,6 +9,7 @@
     "-Wno-conversion",
     "-Wno-implicit-fallthrough",
     "-Wno-sign-compare",
+    "-Wno-write-strings",
   ]
 }