prosody: needs MMU

prosody: needs MMU

prosody POSIX support code calls fork() unconditionally.

prosody also uses the mallinfo() API, but uClibc does not provide that
for no-MMU targets.

Fixes:
http://autobuild.buildroot.net/results/2d0/2d0f97e41cc1d4c9c7a0eacd4eec8bad6f00bcff/
http://autobuild.buildroot.net/results/f69/f69dfb1a7d1163b402b1f9bef045891c229539e5/

Cc: Dushara Jayasinghe <nidujay@gmail.com>
Cc: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>