postgresql: uses fork(), needs mmu

Additional revert 'postgresql: enable bfin compile' ([2]) which
is now useless (Blackfin is a noMMU architecture).

Fixes ([1]):
  postmaster/fork_process.o: In function `_fork_process':
  fork_process.c:(.text+0x22): undefined reference to `_fork'

[1] http://autobuild.buildroot.org/results/c9c/c9c93156fe64e89f35627aa592c47d8c5181ed7d
[2] http://git.buildroot.net/buildroot/commit/?id=6f177dac73a289db28772fb3e20347aba5cd5e67



[Thomas: propagate to the appropriate reverse dependencies.]

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>