lua: fix pkg-config file

When Lua is linked with additional libraries, these libraries should go
into the pkg-config file as well.

Otherwise, linking swupdate with the lua library fails:

```
/home/buildroot/output/host/arm-buildroot-linux-gnueabihf/sysroot/usr/lib/liblua.so: undefined reference to `dlopen'
/home/buildroot/output/host/arm-buildroot-linux-gnueabihf/sysroot/usr/lib/liblua.so: undefined reference to `dlclose'
/home/buildroot/output/host/arm-buildroot-linux-gnueabihf/sysroot/usr/lib/liblua.so: undefined reference to `dlerror'
/home/buildroot/output/host/arm-buildroot-linux-gnueabihf/sysroot/usr/lib/liblua.so: undefined reference to `dlsym'
```

Fixes http://autobuild.buildroot.net/results/1c3/1c349cc5904868e4def292b9fbfa164828e46156



Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>