Add guarded ssl_msg.c to Makefiles

This commit adds the newly created copy ssl_msg.c of ssl_tls.c
to the build system but guards its content by an `#if 0 ... #endif`
preprocessor guard in order to avoid compilation failures resulting
from code duplication. This guard will be removed once the contents
of ssl_tls.c and ssl_msg.c have been made disjoint.

library/ssl_msg.c

@@ -1,3 +1,5 @@
+#if 0
+
 /*
  *  SSLv3/TLSv1 shared functions
  *
@@ -12949,3 +12951,5 @@ exit:
           MBEDTLS_SSL_PROTO_TLS1_2 */
 
 #endif /* MBEDTLS_SSL_TLS_C */
+
+#endif /* 0 */