Fix windows build.

Windows doesn't like struct literals:
..\decrepit\dsa\dsa_decrepit.c(85) : warning C4204: nonstandard extension used : non-constant aggregate initializer

Change-Id: I12541f2883ecbb10c85cddfae8d2adbbb1365ae3
Reviewed-on: https://boringssl-review.googlesource.com/7364
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/decrepit/dsa/dsa_decrepit.c b/decrepit/dsa/dsa_decrepit.c
index 03128b5..aef8056 100644
--- a/decrepit/dsa/dsa_decrepit.c
+++ b/decrepit/dsa/dsa_decrepit.c
@@ -82,8 +82,12 @@
   BN_GENCB gencb_storage;
   BN_GENCB *cb = NULL;
 
-  struct wrapped_callback wrapped = {callback, cb_arg};
+  struct wrapped_callback wrapped;
+
   if (callback != NULL) {
+    wrapped.callback = callback;
+    wrapped.arg = cb_arg;
+
     cb = &gencb_storage;
     BN_GENCB_set(cb, callback_wrapper, &wrapped);
   }