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>

arch/m68k/68000/dragen2.c

@@ -11,6 +11,7 @@
 #include <linux/init.h>
 #include <asm/machdep.h>
 #include <asm/MC68VZ328.h>
+#include "m68328.h"
 #include "screen.h"
 
 /***************************************************************************/