[openweave-core] Promote mac exception to public

Targets that depend on openweave-core need this exception handling
propagated up to them. Promoting the exception to a public config.

Bug: 48083
Test: Built and ran locally.
Change-Id: I7aa9a68aa776591220cb48e892fff567a512dd11
diff --git a/src/include/BUILD.gn b/src/include/BUILD.gn
index 2df0672..1e4f8e1 100644
--- a/src/include/BUILD.gn
+++ b/src/include/BUILD.gn
@@ -7,6 +7,11 @@
 config("public") {
   include_dirs = [ "$target_gen_dir" ]
   cflags = [ "-includeBuildConfig.h" ]
+  # Once the file generation in mainline is fixed, we can remove this exception.
+  # Until then, certain generated files have lower-case instead of upper-case paths.
+  if (host_os == "mac") {
+    cflags += [ "-Wno-nonportable-include-path" ]
+  }
 }
 
 # Common compilation flags for all components in OpenWeave.
@@ -19,11 +24,6 @@
     "-Wno-unused-const-variable",
     "-Wno-unused-function",
   ]
-  # Once the file generation in mainline is fixed, we can remove this exception.
-  # Until then, certain generated files have lower-case instead of upper-case paths.
-  if (host_os == "mac") {
-    cflags += [ "-Wno-nonportable-include-path" ]
-  }
 }
 
 # OpenWeave installs headers into namespaces that don't map directly to their