Fix building on uclibc

While building on uclibc this error is thrown:
In file included from ./include/dns/log.h:20,
                 from callbacks.c:19:
../../lib/isc/include/isc/log.h:141:9: error: unknown type name ‘off_t’
  141 |         off_t maximum_size;
      |         ^~~~~

This is due to missing include unistd.h, so let's add it on top of
isc/log.h

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
(cherry picked from commit 0e43f62c12)

lib/isc/include/isc/log.h

@@ -19,6 +19,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 #include <syslog.h> /* XXXDCL NT */
+#include <unistd.h>
 
 #include <isc/formatcheck.h>
 #include <isc/lang.h>