BUILD.gn: only use complete_static_lib on mac

BUILD.gn

@@ -157,7 +157,11 @@ static_library("glslang_static") {
   deps = [
     ":glslang_sources",
   ]
-  complete_static_lib = true
+
+  # Without this the macOS linker complains that the static library is empty
+  if (is_mac) {
+    complete_static_lib = true
+  }
 
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]