x86/crash: Add a forward declaration of struct kimage

Add a forward declaration of struct kimage to the crash.h header because
future changes will invoke a crash-specific function from the realmode
init path and the compiler will complain otherwise like this:

  In file included from arch/x86/realmode/init.c:11:
  ./arch/x86/include/asm/crash.h:5:32: warning: ‘struct kimage’ declared inside\
   parameter list will not be visible outside of this definition or declaration
      5 | int crash_load_segments(struct kimage *image);
        |                                ^~~~~~
  ./arch/x86/include/asm/crash.h:6:37: warning: ‘struct kimage’ declared inside\
   parameter list will not be visible outside of this definition or declaration
      6 | int crash_copy_backup_region(struct kimage *image);
        |                                     ^~~~~~
  ./arch/x86/include/asm/crash.h:7:39: warning: ‘struct kimage’ declared inside\
   parameter list will not be visible outside of this definition or declaration
      7 | int crash_setup_memmap_entries(struct kimage *image,
        |

 [ bp: Rewrite the commit message. ]

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Lianbo Jiang <lijiang@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: bhe@redhat.com
Cc: d.hatayama@fujitsu.com
Cc: dhowells@redhat.com
Cc: dyoung@redhat.com
Cc: ebiederm@xmission.com
Cc: horms@verge.net.au
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jürgen Gross <jgross@suse.com>
Cc: kexec@lists.infradead.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: vgoyal@redhat.com
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20191108090027.11082-4-lijiang@redhat.com
Link: https://lkml.kernel.org/r/201910310233.EJRtTMWP%25lkp@intel.com

arch/x86/include/asm/crash.h

@@ -2,6 +2,8 @@
 #ifndef _ASM_X86_CRASH_H
 #define _ASM_X86_CRASH_H
 
+struct kimage;
+
 int crash_load_segments(struct kimage *image);
 int crash_copy_backup_region(struct kimage *image);
 int crash_setup_memmap_entries(struct kimage *image,