m68k: kernel: Add and use "process.h"

When building with W=1:

    arch/m68k/kernel/process.c:115:16: warning: no previous prototype for ‘m68k_clone’ [-Wmissing-prototypes]
      115 | asmlinkage int m68k_clone(struct pt_regs *regs)
	  |                ^~~~~~~~~~
    arch/m68k/kernel/process.c:136:16: warning: no previous prototype for ‘m68k_clone3’ [-Wmissing-prototypes]
      136 | asmlinkage int m68k_clone3(struct pt_regs *regs)
	  |                ^~~~~~~~~~~

Fix this by introducing a new header file "process.h" for holding the
prototypes of functions implemented in arch/m68k/kernel/process.c.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/5e50257d8fcae3eb202ce5f439dc29c09cb6c44f.1694613528.git.geert@linux-m68k.org

arch/m68k/kernel/process.c

@@ -38,6 +38,7 @@
 #include <asm/machdep.h>
 #include <asm/setup.h>
 
+#include "process.h"
 
 asmlinkage void ret_from_fork(void);
 asmlinkage void ret_from_kernel_thread(void);

arch/m68k/kernel/process.h

+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <linux/linkage.h>
+
+struct pt_regs;
+
+asmlinkage int m68k_clone(struct pt_regs *regs);
+asmlinkage int m68k_clone3(struct pt_regs *regs);