[libc] Fix test case that expects time_t to be a 32-bit type

This patch changes a test case that tests for overflow when time_t is
32-bit long, however, it was checking size_t instead of time_t.

This in on par with other testcases that correctly check the size of
time_t (asctime_test.cpp, gmtime_r_test.cpp and gmtime_test.cpp).

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D159113