[gyp] fix broken source list for openbsd.

R=machenbach@chromium.org

Fixes https://github.com/nodejs/node/issues/15784

Change-Id: I4453023a5a6c8ace45b898ea227870fb6cbc20b2
Reviewed-on: https://chromium-review.googlesource.com/893261
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51017}

gypfiles/v8.gyp

@@ -2061,9 +2061,10 @@
                 '-L/usr/local/lib -lexecinfo',
             ]},
             'sources': [
+              '../src/base/debug/stack_trace_posix.cc',
               '../src/base/platform/platform-openbsd.cc',
               '../src/base/platform/platform-posix.h',
-              '../src/base/platform/platform-posix.cc'
+              '../src/base/platform/platform-posix.cc',
               '../src/base/platform/platform-posix-time.h',
               '../src/base/platform/platform-posix-time.cc',
             ],