aircrack-ng: disable on non-MMU systems

aircrack-ng uses fork() and therefore fails to build on non-MMU
systems:

aircrack-ng.o: In function `_clean_exit':
aircrack-ng.c:(.text+0x555c): undefined reference to `_fork'
collect2: ld returned 1 exit status

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