build: Add -Wno-unused-parameter compiler option

Due to being a compositor, kwin has to conform to some certain interfaces. It means a lot of virtual functions and function tables to integrate with C APIs. Naturally, we not always want to use every argument in such functions.

Since we get -Wunused-parameter from -Wall, we have to plumb those unused arguments in order to suppress compiler warnings at the moment.

However, I don't think that extra work is worth it. We cannot change or alter prototypes in any way to fix the warning the desired way. Q_UNUSED and similar macros are not good indicators of whether an argument is used too, we tend to overlook putting or removing those macros. I've also noticed that Q_UNUSED are not used to guide us with the removal no longer needed parameters.

Therefore, I think it's worth adding -Wno-unused-parameter compiler option to stop the compiler producing warnings about unused parameters. It changes nothing except that we don't need to put Q_UNUSED anymore, which can be really cumbersome sometimes. Note that it doesn't affect unused variables, you'll still get a -Wunused-variable compiler warning if a variable is unused.