pw_build: Correctly append polyfill dependency

This fixes a bug in pw_build's global public depends where it was
overriding instead of appending polyfill.

Change-Id: I3d1384029f0587c20236b3ff0cbe0c10883c1e6d
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/18340
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Alexei Frolov <frolv@google.com>
diff --git a/pw_build/defaults.gni b/pw_build/defaults.gni
index 5a481f4..5c2077e 100644
--- a/pw_build/defaults.gni
+++ b/pw_build/defaults.gni
@@ -40,7 +40,7 @@
     "$dir_pw_build:strict_warnings",
     "$dir_pw_build:cpp17",
   ]
-  public_deps = [ "$dir_pw_polyfill:overrides" ]
+  public_deps += [ "$dir_pw_polyfill:overrides" ]
 }
 
 # One more pass, to remove configs