Fix UWP build

Bug: skia:11102
Change-Id: Id7f91da15d3021ebe9acad968165521a7d590fed
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/372162
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 3f82ffb..dd12bd9 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -93,7 +93,7 @@
     defines += [ "SK_USE_VMA" ]
   }
   if (skia_enable_winuwp) {
-    defines = [ "SK_WINUWP" ]
+    defines += [ "SK_WINUWP" ]
   }
 }