net: lib: ocpp: use sys_clock_gettime() instead of gettimeofday()

Use the native zephyr sys_clock_gettime() call instead of the XSI
gettimeofday(), since XSI is not required here and would normally
require CONFIG_XSI_SINGLE_PROCESS to be selected.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>