m68k: mm: Include <asm/hwtest.h> for hwreg_()

When building with W=1:

    arch/m68k/mm/hwtest.c:29:5: warning: no previous prototype for ‘hwreg_present’ [-Wmissing-prototypes]
       29 | int hwreg_present(volatile void *regp)
	  |     ^~~~~~~~~~~~~
    arch/m68k/mm/hwtest.c:62:5: warning: no previous prototype for ‘hwreg_write’ [-Wmissing-prototypes]
       62 | int hwreg_write(volatile void *regp, unsigned short val)
	  |     ^~~~~~~~~~~

Fix this by including <asm/hwtest.h>.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/fbd87d8e8d1e8cbe7d56941a8a1d7d82b53010d0.1694613528.git.geert@linux-m68k.org

arch/m68k/mm/hwtest.c

@@ -26,6 +26,8 @@
 
 #include <linux/module.h>
 
+#include <asm/hwtest.h>
+
 int hwreg_present(volatile void *regp)
 {
 	int ret = 0;