firmware-utils: mkdlinkfw: fix build

firmware-utils: mkdlinkfw: fix build

While we don't need the gnu99 option anymore, we still need to force the
c99 standard to fix the following build error on the build bots:

src/mkdlinkfw.c: In function 'find_auh_headers':
src/mkdlinkfw.c:267:3: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
   for (int i = 0; i < header_counter; i++) {
   ^
src/mkdlinkfw.c:267:3: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code

Signed-off-by: Mathias Kresin <dev@kresin.me>