buildroot

apr: fix size of pid_t

pid_t is a signed 32bits integer on both 32bits and 64bits
architectures.
This fixes an issue with apache server which causes bad pid
to be written in PidFile

Signed-off-by: Julien Beraud <julien.beraud@spectracom.orolia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>