g++ cannot compile <utmpx.h> after time_t merge

[Tobias Nygren <tnn%NetBSD.org@localhost>]

This one bit me while rebuilding pkgsrc/sysutils/smartmontools:

> echo "#include <utmpx.h>" > test.cpp
> g++ -c -o test.o test.cpp
/usr/include/utmpx.h:107: error: types may not be defined in 'sizeof'
expressions