gnu: gfortran: change to a hidden package

* gnu/packages/gcc.scm (gfortran-4.8, gfortran-4.9)
(gfortran-5, gfortran-6, gfortran-7, gfortran-8)
(gfortran-9): Wrap in 'hidden-package'.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>