Work around MSVC's limitations.

MSVC 2013 does not support constexpr:
https://msdn.microsoft.com/en-us/library/vstudio/hh567368.aspx

Change-Id: I73a98ace57356489efeb25384997d2d2891a271f
diff --git a/ssl/test/bssl_shim.cc b/ssl/test/bssl_shim.cc
index c0d5475..5fd97ca 100644
--- a/ssl/test/bssl_shim.cc
+++ b/ssl/test/bssl_shim.cc
@@ -473,7 +473,7 @@
 
 // kCustomExtensionValue is the extension value that the custom extension
 // callbacks will add.
-constexpr uint16_t kCustomExtensionValue = 1234;
+static const uint16_t kCustomExtensionValue = 1234;
 static void *const kCustomExtensionAddArg =
     reinterpret_cast<void *>(kCustomExtensionValue);
 static void *const kCustomExtensionParseArg =