elf coredump: include <linux/elfcore.h> to fix warnings

The kernel/elfcore.c defines functions declared in the header <linux/elfcore.h> but does not current include it. The inclusion fixes the following sparse warnings:

kernel/elfcore.c:6:17: warning: symbol 'elf_core_extra_phdrs' was not declared. Should it be static? kernel/elfcore.c:11:12: warning: symbol 'elf_core_write_extra_phdrs' was not declared. Should it be static? kernel/elfcore.c:16:12: warning: symbol 'elf_core_write_extra_data' was not declared. Should it be static? kernel/elfcore.c:21:15: warning: symbol 'elf_core_extra_data_size' was not declared. Should it be static?

---
Cc: [email protected]

---
 kernel/elfcore.c | 1 +
 1 file changed, 1 insertion(+)

index e556751..a2b29b9 100644
@@ -2,6 +2,7 @@
 #include <linux/fs.h>
 #include <linux/mm.h>
 #include <linux/binfmts.h>
+#include <linux/elfcore.h>
 
 Elf_Half __weak elf_core_extra_phdrs(void)
 {

2.8.1