luarocks: use -fPIC for shared library

Fixes:
http://autobuild.buildroot.net/results/f28a624f89f4de249a1b7a718f5ae31109ba9171/
http://autobuild.buildroot.net/results/1ead0a162a251d3e7bf8598036fa06c4befeb3d9/
http://autobuild.buildroot.net/results/0544cead086ba51b45dfa1df114787a8f22eaf8d/
http://autobuild.buildroot.net/results/60161d13d596d41dd4059977eb947d72c172f0d5/
http://autobuild.buildroot.net/results/e7e/e7ea6946552f72dc27f23ae1aff1665a6aa097c7/

[Peter: adjust commit message]
Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>