testsuite, fortran: add required external keyword

testsuite, fortran: add required external keyword

Currenlty, ifx/ifort cannot compile the given executable as it is not
valid Fortran.  It is missing the external keyword on the
no_arg_subroutine.  Gfortran compiles the example but this is actually
a bug and there is an open gcc ticket for this here:

   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50377

Adding the keyword does not change the gfortran compiling of the example.
It will, however, prevent a future fail once 50377 has been addressed.