configure: add SIZEOF_TIME_T to public_config.h

configure: add SIZEOF_TIME_T to public_config.h

This has recently started being used by ext2fs.h.  Other users of the
ext2fs.h header would always get the 32-bit versions of the
__encode_extra_time, __decode_extra_sec, and __sb_set_tstamp; and the
64-bit version of __sb_get_tstamp, due to the SIZEOF_TIME_T macro
being undefined and treated as zero.

Fixes: 5b599a32 ("Fix 32-bit build and test failures")
Signed-off-by: Alyssa Ross <hi@alyssa.is>
Link: https://lore.kernel.org/r/20240527074121.2767083-1-hi@alyssa.is
Signed-off-by: Theodore Ts'o <tytso@mit.edu>