Merge fix for zfs readmmap test from CheriBSD
Merge commit 1737d8397a0 by Brooks Davis:
time() is declared in time.h
This fixes a -Werror warning from clang 15:
tests/sys/cddl/zfs/bin/readmmap.c:97:9: error: call to undeclared function 'time'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
Obtained from: https://github.com/CTSRD-CHERI/cheribsd/commit/1737d8397a0
MFC after: 3 days