GN: add missing .exe

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2168763005

NOTRY=true

Review-Url: https://codereview.chromium.org/2168763005
diff --git a/DEPS b/DEPS
index 5964f26..ee605b2 100644
--- a/DEPS
+++ b/DEPS
@@ -90,7 +90,7 @@
                '--no_auth',
                '--bucket', 'chromium-gn',
                '--platform=win32',
-               '-s', 'skia/buildtools/win/gn.sha1'],
+               '-s', 'skia/buildtools/win/gn.exe.sha1'],
 }]
 
 recursedeps = [ "common" ]