iwyu: Cleanup include headers

Triggered iwyu and add/remove used headers.

Change-Id: Iadbdb459563b3c89baf2d56f90a540342d020cd5
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/subprojects/ncsid/test/net_iface_mock.h b/subprojects/ncsid/test/net_iface_mock.h
index 0ccf5bf..118ca03 100644
--- a/subprojects/ncsid/test/net_iface_mock.h
+++ b/subprojects/ncsid/test/net_iface_mock.h
@@ -18,6 +18,7 @@
 
 #include <net_iface.h>
 
+#include <string>
 #include <vector>
 
 namespace mock