pc-bios/s390-ccw: Add a proper prototype for main()

pc-bios/s390-ccw: Add a proper prototype for main()

Older versions of Clang complain if there is no prototype for main().
Add one, and while we're at it, make sure that we use the same type
for main.c and netmain.c - since the return value does not matter,
declare the return type of main() as "void".

Message-Id: <20220704111903.62400-2-thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>