#42 Undefined UNUSEDARG macro when compiling in Linux

[SilverNexus]

When compiling in Linux, the UNUSEDARG macro used in Twain_App_sample01/src/main.cpp is not defined.
This can be solved by defining UNUSEDARG in common/Common.h.

Attached is a diff of how I made this work. Probably needs testing to ensure this doesn't break other builds.