include/tc_util.h: Rename __str to fix C++ compile error

Several C++ std library headers use __str as internal
variable names. If those headers are included after
tc_util.h is included those headers fail to compile
because tc_util.h defined __str to be a macro.

Fixed by renaming the __str macro to TC_STR.

Signed-off-by: Erwin Rol <erwin@erwinrol.com>