file: fix call to localtime_r()

Depending on the version of glibc, localtime_r() must
be preceded by a call to tzset() or it will ignore any
value of TZ in the environment.  This problem will only
be seen when building file-native on outdated hosts.

Signed-off-by: Joe Slater <joe.slater@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>