package/collectd: fix build with luajit

Build with luajit has been enabled with commit
911a9bf5 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>