heirloom-mailx: needs fork()

heirloom-mailx uses fork(), so we can't use it as is on !mmu
platforms.

Fixes:

http://autobuild.buildroot.org/results/c5a8e3a4cbc1e1de985fe3c724b711027865abe7/build-end.log



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