Fix for build and link errors in some platform

Makefile.PL: make sure -lqrencode option is put AFTER object and source files that use libqrencode.

This is necessary because otherwise gcc could not find symbols in libqrencode, causing a linker error.
I confirmed the error on Ubuntu 12.04 32bit, gcc 4.6.3

This patch should fix the bugs reported in CPAN RT:

- https://rt.cpan.org/Public/Bug/Display.html?id=79850
- https://rt.cpan.org/Public/Bug/Display.html?id=82932

Note that I did not do any regression testing.