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

[mikhail.ramalho]

Summary

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).

libc/test/src/time/mktime_test.cpp

EXPECT_TM_EQ((tm{.tm_sec = 0,
                    .tm_mon = Month::JANUARY,
                    .tm_year = tm_year(1969),
                    .tm_wday = 3,
                    .tm_yday = 0}),
                tm_data);
 }
 
 TEST(LlvmLibcMkTime, InvalidEndOf32BitEpochYear) {
-  if (sizeof(size_t) != 4)
+  if (sizeof(time_t) != 4)
     return;
   {
     // 2038-01-19 03:14:08 tests overflow of the second in 2038.
     struct tm tm_data {
       .tm_sec = 8, .tm_min = 14, .tm_hour = 3, .tm_mday = 19,
       .tm_mon = Month::JANUARY, .tm_year = tm_year(2038), .tm_wday = 0,
       .tm_yday = 0
     };