Include <time.h> from netdissect.h.

Include <time.h> from netdissect.h.

netdissect.h since commit 7578e1c requires <time.h> for struct tm.  The
dependency is difficult to notice because the warning pops up when
compiling strdup.c only and only on Linux, and Linux most certainly has
strdup(), so normally strdup.c is never compiled on Linux.  Make it
correct anyway to eliminate a space for future surprises in other files.

$ make strdup.o
In file included from ./missing/strdup.c:38:
./netdissect.h:441:38: warning: 'struct tm' declared inside parameter
  list will not be visible outside of this definition or declaration

gcc (Debian 10.2.1-6) 10.2.1 20210110