x86/purgatory: Include header for warn() declaration

The purgatory code uses parts of the decompressor and provides its own
warn() function, but has to include the corresponding header file to
avoid a -Wmissing-prototypes warning.

It turns out that this function prototype actually differs from the
declaration, so change it to get a constant pointer in the declaration
and the other definition as well.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230803082619.1369127-5-arnd@kernel.org

arch/x86/boot/compressed/error.c

@@ -7,7 +7,7 @@
 #include "misc.h"
 #include "error.h"
 
-void warn(char *m)
+void warn(const char *m)
 {
 	error_putstr("\n\n");
 	error_putstr(m);

arch/x86/boot/compressed/error.h

@@ -4,7 +4,7 @@
 
 #include <linux/compiler.h>
 
-void warn(char *m);
+void warn(const char *m);
 void error(char *m) __noreturn;
 void panic(const char *fmt, ...) __noreturn __cold;
 

arch/x86/purgatory/purgatory.c

@@ -14,6 +14,7 @@
 #include <crypto/sha2.h>
 #include <asm/purgatory.h>
 
+#include "../boot/compressed/error.h"
 #include "../boot/string.h"
 
 u8 purgatory_sha256_digest[SHA256_DIGEST_SIZE] __section(".kexec-purgatory");