wpa_supplicant: uses fork(), not available on no-MMU platforms

The only package selecting wpa_supplicant is connman, and it is
already depends on BR2_USE_MMU.

Fixes:

  http://autobuild.buildroot.org/results/9ac/9ac3c0de573f5266a889f1ceb6d43aa92f715c55/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>