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

tests/sys/cddl/zfs/bin/readmmap.c

@@ -43,6 +43,7 @@
  */
 #include <stdio.h>
 #include <stdlib.h>
+#include <time.h>
 #include <unistd.h>
 #include <fcntl.h>
 #include <errno.h>