Fix build of x509_test.

Some combination of Chromium's copy of clang and Chromium's Linux sysroot
doesn't like syntax. It complains that "chosen constructor is explicit in
copy-initialization".

Change-Id: Ied6bc17b19421998f926483742510c81f732566b
Reviewed-on: https://boringssl-review.googlesource.com/6930
Reviewed-by: Adam Langley <agl@google.com>

crypto/x509/x509_test.cc

diff --git a/crypto/x509/x509_test.cc b/crypto/x509/x509_test.cc
index b5769cc..486fc4c 100644
--- a/crypto/x509/x509_test.cc
+++ b/crypto/x509/x509_test.cc
@@ -239,12 +239,13 @@
     return 1;
   }
 
-  if (Verify(leaf.get(), {}, {})) {
+  std::vector<X509*> empty;
+  if (Verify(leaf.get(), empty, empty)) {
     fprintf(stderr, "Leaf verified with no roots!\n");
     return 1;
   }
 
-  if (Verify(leaf.get(), {}, {intermediate.get()})) {
+  if (Verify(leaf.get(), empty, {intermediate.get()})) {
     fprintf(stderr, "Leaf verified with no roots!\n");
     return 1;
   }