apr: needs mmu

Uses fork() in apr_proc_fork() which is used by almost all the packages
that use apr (log4cxx, subversion).
apr-util doesn't use fork or apr_proc_fork but it's of no use alone.

[Peter: also hide log4cxx comment if !BR2_USE_MMU]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>