netplug: disable -Werror to fix build failure

netplug uses nested functions, and gcc generates a warning that
is turned into an error:

cc1: warnings being treated as errors
if_info.c: In function 'ifsm_scriptdone':
if_info.c:289: error: generating trampoline in object (requires executable stack)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>