glx,xprint: place explicit padding between lists

Adjacent lists of char should not have any automatic padding added, since the alignment requirements for both are the same. Add explicit padding where it is required.

Since libxcb has a bug that inserts padding where it shouldn't, this doesn't change the effect of libxcb generated code for serialization today, but it does fix the generated _sizeof functions. This is also a prerequisite for fixing the bug in libxcb.