Fix Win GN error in mesa's file.

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Review URL: https://codereview.chromium.org/707733006

Cr-Commit-Position: refs/heads/master@{#303346}

Author: John Abd-El-Malek

third_party/mesa/BUILD.gn

@@ -113,7 +113,7 @@ config("mesa_internal_config") {
   if (is_win) {
     # TODO(scottmg): http://crbug.com/143877 These should be removed if
     # Mesa is ever rolled and the warnings are fixed.
-    cflags = [
+    cflags += [
       "/wd4005",  # Macro defined twice.
       "/wd4065",  # Switch statement contains 'default' but no 'case' labels.
       "/wd4090",  # 'Operation' : different 'modifier' qualifiers