aircrack-ng: fix build with mmx

Commit 39387fc80f90f3a9ac9ef9f3aa32da5776a0721e removed mmx support
however aircrack-ng fails to build on platforms with mmx because an
error is raised if __MMX__ is defined.

Fixes:
 - http://autobuild.buildroot.net/results/b7362b69435e9ef6fb2aedc50743e88dbd7a5c72

[Upstream status: merged
(https://github.com/aircrack-ng/aircrack-ng/pull/1943)

]

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>