package/collectd: fix build with luajit

Build with luajit has been enabled with commit
911a9bf5739d94545cc482c765f876a9ff73e304 however the build fails because
collectd does not find liblua

Fixes:
 - http://autobuild.buildroot.org/results/31302b47fb70eb442cd000f4b9f27a9e12432cc3

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/collectd/collectd.mk

@@ -198,6 +198,9 @@ endif
 ifeq ($(BR2_PACKAGE_LIBCURL),y)
 COLLECTD_CONF_OPTS += --with-libcurl=$(STAGING_DIR)/usr
 endif
+ifeq ($(BR2_PACKAGE_LUAJIT),y)
+COLLECTD_CONF_ENV += LIBLUA_PKG_CONFIG_NAME=luajit
+endif
 ifeq ($(BR2_PACKAGE_MYSQL),y)
 COLLECTD_CONF_OPTS += --with-libmysql=$(STAGING_DIR)/usr
 endif