Include the type in empty vector literals.

Chromium on Linux builds against libstdc++'s debug mode which makes
clang unhappy due to:

../crypto/bytestring/bytestring_test.cc:910:7: error: chosen constructor
is explicit in copy-initialization
      {},
      ^~
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/debug/vector:79:7:
note: constructor declared here
      vector(const _Allocator& __a = _Allocator())
      ^

I believe this was fixed here, but it's too recent:
https://github.com/gcc-mirror/gcc/commit/36f540c70ba27e441bd07111a2107b8993382905

Change-Id: I2942d153e1278785c3b81294bc99b86f297cf719
Reviewed-on: https://boringssl-review.googlesource.com/12967
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/bytestring/bytestring_test.cc b/crypto/bytestring/bytestring_test.cc
index b2b20d7..12275d6 100644
--- a/crypto/bytestring/bytestring_test.cc
+++ b/crypto/bytestring/bytestring_test.cc
@@ -907,7 +907,7 @@
 
   static const std::vector<uint8_t> kInvalidBitStrings[] = {
       // BIT STRINGs always have a leading byte.
-      {},
+      std::vector<uint8_t>{},
       // It's not possible to take an unused bit off the empty string.
       {0x01},
       // There can be at most 7 unused bits.