Add missing md.h includes

MBEDTLS_MD_CAN_SHAXXX are defined there.
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 7d526a7..84c4bef 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -40,6 +40,8 @@
 #include "mbedtls/dhm.h"
 #endif
 
+#include "mbedtls/md.h"
+
 #if defined(MBEDTLS_ECDH_C)
 #include "mbedtls/ecdh.h"
 #endif