[Wconversion] Suppress warnings

Fix issue in commit 722fa7ebbee6b415596c2879d916600ee43c1a90.

Bug: 56258
Change-Id: Ie3faef8bb009f56474d44014f1f16a0b15a0cf3a
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/gflags/+/423620
Reviewed-by: Petr Hosek <phosek@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 0f0a7b6..ee1e6d2 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -38,6 +38,6 @@
     "src/gflags_completions.cc",
     "src/gflags_reporting.cc",
   ]
+  cflags = [ "-Wno-conversion" ]
   public_configs = [ ":gflags_config" ]
-  configs += [ "//build/config:Wno-conversion" ]
 }