x86/purgatory: Fix sparse warning, symbol not declared

Sparse emits warning, 'symbol not declared' for a function that has
neither file scope nor a forward declaration. The functions only call
site is an ASM file.

Add a header file with the function declaration. Include the header file in
the C source file defining the function in order to fix the sparse
warning. Include the header file in ASM file containing the call site to
document the usage.
Signed-off-by: Tobin C. Harding <me@tobin.cc>
Link: http://lkml.kernel.org/r/1487545956-2547-3-git-send-email-me@tobin.ccSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/purgatory/purgatory.c

@@ -11,6 +11,7 @@
  */
 
 #include "sha256.h"
+#include "purgatory.h"
 #include "../boot/string.h"
 
 struct sha_region {

arch/x86/purgatory/purgatory.h

+#ifndef PURGATORY_H
+#define PURGATORY_H
+
+#ifndef __ASSEMBLY__
+extern void purgatory(void);
+#endif	/* __ASSEMBLY__ */
+
+#endif /* PURGATORY_H */

arch/x86/purgatory/setup-x86_64.S

@@ -9,6 +9,7 @@
  * This source code is licensed under the GNU General Public License,
  * Version 2.  See the file COPYING for more details.
  */
+#include "purgatory.h"
 
 	.text
 	.globl purgatory_start