package/apcupsd: set ac_cv_path_SHUTDOWN

Set ac_cv_path_SHUTDOWN to /sbin/shutdown to avoid the following build
failure when shutdown is not available on host raised since the addition
of the package in commit d12b63b741586b89c4017551308c43512a617287:

configure: error: Missing required tool; need any one of: shutdown shutdown.bsd

Fixes: d12b63b741586b89c4017551308c43512a617287
 - http://autobuild.buildroot.org/results/a8bbb5ced8343e08070361f260050de422144710

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/apcupsd/apcupsd.mk

@@ -11,6 +11,7 @@ APCUPSD_LICENSE_FILES = COPYING
 APCUPSD_CPE_ID_VENDOR = apcupsd
 APCUPSD_SELINUX_MODULES = apache apcupsd
 APCUPSD_CONF_OPTS = --disable-test
+APCUPSD_CONF_ENV = ac_cv_path_SHUTDOWN=/sbin/shutdown
 
 ifneq ($(BR2_PACKAGE_APCUPSD_MODBUS_USB)$(BR2_PACKAGE_APCUPSD_USB),)
 APCUPSD_CONF_ENV += ac_cv_path_usbcfg=$(STAGING_DIR)/usr/bin/libusb-config