clang/win: Attempt to fix bots after https://codereview.chromium.org/1275833002

Should fix:
..\..\chrome\browser\media\encrypted_media_browsertest.cc(42,12) :
    error: unused variable 'kExternalClearKeyDecryptOnlyKeySystem'
           [-Werror,-Wunused-const-variable]
const char kExternalClearKeyDecryptOnlyKeySystem[] =
           ^
BUG=82385
TBR=jam@chromium.org

Review URL: https://codereview.chromium.org/1280893002.

Cr-Commit-Position: refs/heads/master@{#342285}
1 file changed