BUILD: listener: fix 'for' loop inline variable declaration

BUILD: listener: fix 'for' loop inline variable declaration

commit 16eb0fab3 ("MAJOR: counters: dispatch counters over thread groups")
introduced a build regression on some compilers:

  src/listener.c: In function 'listener_accept':
  src/listener.c:1095:3: error: 'for' loop initial declarations are only allowed in C99 mode
     for (int it = 0; it < global.nbtgroups; it++)
     ^
  src/listener.c:1095:3: note: use option -std=c99 or -std=gnu99 to compile your code
  src/listener.c:1101:4: error: 'for' loop initial declarations are only allowed in C99 mode
      for (int it = 0; it < global.nbtgroups; it++) {
      ^
  make: *** [src/listener.o] Error 1
  make: *** Waiting for unfinished jobs....

Let's fix that.
No backport needed