firmware-utils: fix nec-enc build on older architectures

firmware-utils: fix nec-enc build on older architectures

This patch enable gnu99 mode for the nec-enc utility which
fixes the following build-breaking errors on some older
architectures.

nec-enc.c: In function ‘xor_data’:
nec-enc.c:34:2: error: ‘for’ loop initial declarations are only allowed in C99 or C11 mode
  for (int i = 0; i < len; i++) {
  ^~~
nec-enc.c:34:2: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code
nec-enc.c: In function ‘main’:
nec-enc.c:101:3: error: ‘for’ loop initial declarations are only allowed in C99 or C11 mode
   for (int i = 0; i < n; i++) {
   ^~~

Spotted-By: Buildbot
Fixes: fac27643f057 ("firmware-utils: add nec-enc")
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>