openssl: re-export the last-resort locking function

We need to include the header where we define the function. Otherwise it
won't be available on the DLL.

src/openssl_stream.c

diff --git a/src/openssl_stream.c b/src/openssl_stream.c
index 58b70f5..1dad5f6 100644
--- a/src/openssl_stream.c
+++ b/src/openssl_stream.c
@@ -15,6 +15,7 @@
 #include "socket_stream.h"
 #include "netops.h"
 #include "git2/transport.h"
+#include "git2/sys/openssl.h"
 
 #ifdef GIT_CURL
 # include "curl_stream.h"