Pass |alice_msg| by reference in the SPAKE2 speed test.

This is an attempt to make MSVC happy. Currently it's saying:

..\tool\speed.cc(508) : error C2536: 'SpeedSPAKE2::<lambda_…>::SpeedSPAKE2::<lambda_…>::alice_msg' : cannot specify explicit initializer for arrays

Change-Id: Ifba1df26b5d734f142668a41834645c1549f9f52
Reviewed-on: https://boringssl-review.googlesource.com/7248
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/tool/speed.cc b/tool/speed.cc
index a4edfe7..9f426e2 100644
--- a/tool/speed.cc
+++ b/tool/speed.cc
@@ -490,7 +490,7 @@
     return false;
   }
 
-  if (!TimeFunction(&results, [alice_msg, alice_msg_len]() -> bool {
+  if (!TimeFunction(&results, [&alice_msg, alice_msg_len]() -> bool {
         ScopedSPAKE2_CTX bob(SPAKE2_CTX_new(spake2_role_bob, kBobName,
                                             sizeof(kBobName), kAliceName,
                                             sizeof(kAliceName)));