GN/Win: pass /DEBUG to linker to get debug symbols.

Not much used for debug symbols if they're not linked in.

CQ_INCLUDE_TRYBOTS=master.client.skia.compile:Build-Win-MSVC-x86-Debug-Exceptions-Trybot,Build-Win-MSVC-x86_64-Debug-GN-Trybot,Build-Win-MSVC-x86_64-Release-GN-Trybot

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3320

Change-Id: I506ae624ec412d4151a025879a5dd14d90183bd8
Reviewed-on: https://skia-review.googlesource.com/3320
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index 625ed73..b4eb327 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -306,6 +306,7 @@
     cflags = [ "-gline-tables-only" ]
   } else if (is_win) {
     cflags = [ "/Zi" ]
+    ldflags = [ "/DEBUG" ]
   } else {
     cflags = [ "-g" ]
   }