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>

package/apr/apr.mk

@@ -22,6 +22,7 @@ APR_CONF_ENV = \
 	apr_cv_mutex_robust_shared=no \
 	apr_cv_tcp_nodelay_with_cork=yes \
 	ac_cv_sizeof_struct_iovec=8 \
+	ac_cv_sizeof_pid_t=4 \
 	ac_cv_struct_rlimit=yes \
 	ac_cv_o_nonblock_inherited=no \
 	apr_cv_mutex_recursive=yes