powerpc/fadump: Fix build break when CONFIG_PROC_VMCORE=n

The fadump code calls vmcore_cleanup() which only exists if
CONFIG_PROC_VMCORE=y. We don't want to depend on CONFIG_PROC_VMCORE,
because it's user selectable, so just wrap the call in an #ifdef.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/fadump.c

@@ -1108,7 +1108,9 @@ static ssize_t fadump_release_memory_store(struct kobject *kobj,
 		 * Take away the '/proc/vmcore'. We are releasing the dump
 		 * memory, hence it will not be valid anymore.
 		 */
+#ifdef CONFIG_PROC_VMCORE
 		vmcore_cleanup();
+#endif
 		fadump_invalidate_release_mem();
 
 	} else