package/civetweb: fix build error on a host with make v4.0

Add a patch to fix a typo that on a system with make v4.0 and
BR2_CIVETWEB_WITH_LUA=y fails to build:
  Makefile:203: *** mixed implicit and normal rules.  Stop.

The patch was not sent upstream because this problem is already fixed on
master.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>