Enable bssl client/s_client and server/s_server on Windows.

Change-Id: Iea9bd25176724b56ebb21bded6925f5d30176548
Reviewed-on: https://boringssl-review.googlesource.com/3071
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/tool/digest.cc b/tool/digest.cc
index 9c75d2d..086d3a4 100644
--- a/tool/digest.cc
+++ b/tool/digest.cc
@@ -37,6 +37,7 @@
 #include <io.h>
 #define PATH_MAX MAX_PATH
 #define read _read
+typedef int ssize_t;
 #endif
 
 #include <openssl/digest.h>