Scripts: kconfig: nconf: fix _GNU_SOURCE redefined warning

Fix below warning when make nconfig is run initially
or after make clean.

  HOSTCC  scripts/kconfig/nconf.o
scripts/kconfig/nconf.c:8:0: warning: "_GNU_SOURCE" redefined
 #define _GNU_SOURCE
 ^
<command-line>:0:0: note: this is the location of the previous definition

Signed-off-by: Cheah Kok Cheong <thrust73@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.com>