TMC: add nullfunc() to tmlib

[nthallen]

I've been playing games with the nullfunc() declaration, trying to create it as a static in the file. The problem with this is that we don't know whether we need the function until after it's used. If it's pre-declared as static, we get a warning if it is not defined, even if it isn't reference. If we define it and don't reference it, that generates a different warning. The simpler thing is to simply add it to tmlib and tm.h. Both are always required anyway.

[nthallen]

This may be pointless if there are other warnings that cannot be suppressed. I would just like to reduce the amount of noise on a standard build.

[nthallen]

This appears to be a non-issue or already resolved.