postgresql: disable spinlocks on openrisc

Just like all non-mainstream architectures, postgresql doesn't provide
its internal spinlock implementation on OpenRISC, so disable it.

Fixes:

  http://autobuild.buildroot.net/results/e9de750e34f10c4a2403696936260ad1547d0d15/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>