Adding windows support for gpr subprocesses.
diff --git a/binding.gyp b/binding.gyp
index 651c2e1..ad8015d 100644
--- a/binding.gyp
+++ b/binding.gyp
@@ -516,6 +516,7 @@
         'src/core/support/string_posix.c',
         'src/core/support/string_win32.c',
         'src/core/support/subprocess_posix.c',
+        'src/core/support/subprocess_windows.c',
         'src/core/support/sync.c',
         'src/core/support/sync_posix.c',
         'src/core/support/sync_win32.c',