Add missing md.h includes

MBEDTLS_MD_CAN_SHAXXX are defined there.
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/programs/pkey/dh_server.c b/programs/pkey/dh_server.c
index 66f7d6f..0e033a5 100644
--- a/programs/pkey/dh_server.c
+++ b/programs/pkey/dh_server.c
@@ -20,6 +20,7 @@
 #include "mbedtls/build_info.h"
 
 #include "mbedtls/platform.h"
+#include "mbedtls/md.h"
 
 #if defined(MBEDTLS_AES_C) && defined(MBEDTLS_DHM_C) && \
     defined(MBEDTLS_ENTROPY_C) && defined(MBEDTLS_NET_C) && \