m68knommu: fix warning: no previous prototype for 'init_dragen2'

Fix the compile time warning:

    arch/m68k/68000/dragen2.c:38:13: warning: no previous prototype for 'init_dragen2' [-Wmissing-prototypes]
      38 | void __init init_dragen2(char *command, int size)
         |             ^~~~~~~~~~~~

There already exists a header file prototype, but that file was not
included in the dragen2.c code.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>
Reported-by: kernel test robot <lkp@intel.com>