dav1d: Fix gn with use_thin_lto=true on mac

With use_thin_lto, needs_stack_alignment is false, so `defines`
wasn't, uh, defined when adding PREFIX to it.

Bug: none
Change-Id: I08ddd80386fce17bfd5b135b6a075d7f3bd0cf12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626774
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843101}

third_party/dav1d/BUILD.gn

@@ -140,8 +140,9 @@ if (enable_nasm) {
       rebase_path("$platform_config_root/config.asm", root_build_dir),
     ]
 
+    defines = []
     if (needs_stack_alignment) {
-      defines = [ "STACK_ALIGNMENT=$stack_alignment" ]
+      defines += [ "STACK_ALIGNMENT=$stack_alignment" ]
     }
 
     # Necessary to ensure macOS symbols end up with a _ prefix.