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 <[email protected]>
Signed-off-by: Lianbo Jiang <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jürgen Gross <[email protected]>
Cc: [email protected]
Cc: Thomas Gleixner <[email protected]>
Cc: Tom Lendacky <[email protected]>
Cc: [email protected]
Cc: x86-ml <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/201910310233.EJRtTMWP%[email protected]

Author: lian-bo

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,