dump: const-qualify the buf of WriteCoreDumpFunction

WriteCoreDumpFunction is a function pointer that points to the function used to
write content in "buf" into core file, so "buf" should be const-qualify.

Signed-off-by: Qiao Nuohan <qiaonuohan@cn.fujitsu.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/dump.c b/dump.c
index 80a9116..42622de 100644
--- a/dump.c
+++ b/dump.c
@@ -99,7 +99,7 @@
     dump_cleanup(s);
 }
 
-static int fd_write_vmcore(void *buf, size_t size, void *opaque)
+static int fd_write_vmcore(const void *buf, size_t size, void *opaque)
 {
     DumpState *s = opaque;
     size_t written_size;