sed(1): Move regex.h to be the last included

sed(1): Move regex.h to be the last included

The regex.h will soon include <wchar.h> which pulls in <stdio.h>.  The
main.c needs _WITH_GETLINE defined before <stdio.h> is included in order
to have getline's prototype available.  When the new regex library is
imported, sed will break unless _WITH_GETLINE is defined sooner or unless
stdio.h is included before regex.h is.  Let's just move regex.h to be
the last included header to avoid our future issue.