aux/tc: Add ASSERTED to unreferenced release build variable
What does this MR do and why?
aux/tc: Add ASSERTED to unreferenced release build variable
Fixes MSVC build error src/gallium/auxiliary/util/u_threaded_context.c(3184): error C4189: 'size': local variable is initialized but not referenced
Fixes: 51ad2691 ("aux/tc: handle stride mismatch during rp-optimized subdata")