Add a missing guard for time.h in net_sockets.c

Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/library/net_sockets.c b/library/net_sockets.c
index 17a9e4a..d1700f3 100644
--- a/library/net_sockets.c
+++ b/library/net_sockets.c
@@ -107,7 +107,9 @@
 
 #include <stdio.h>
 
+#if defined(MBEDTLS_HAVE_TIME)
 #include <time.h>
+#endif
 
 #include <stdint.h>