dump: Fix build with newer gcc

gcc 8 on rawhide is picky enough to complain:

/home/dummy/qemu/dump.c: In function 'create_header32':
/home/dummy/qemu/dump.c:817:5: error: 'strncpy' output truncated before terminating nul copying 8 bytes from a string of the same length [-Werror=stringop-truncation]
     strncpy(dh->signature, KDUMP_SIGNATURE, strlen(KDUMP_SIGNATURE));
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

But we already have SIG_LEN defined as the right length without needing
to do a strlen(), and memcpy() is better than strncpy() when we know
we do not want a trailing NUL byte.
Signed-off-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

dump.c

@@ -814,7 +814,7 @@ static void create_header32(DumpState *s, Error **errp)
     size = sizeof(DiskDumpHeader32);
     dh = g_malloc0(size);
 
-    strncpy(dh->signature, KDUMP_SIGNATURE, strlen(KDUMP_SIGNATURE));
+    memcpy(dh->signature, KDUMP_SIGNATURE, SIG_LEN);
     dh->header_version = cpu_to_dump32(s, 6);
     block_size = s->dump_info.page_size;
     dh->block_size = cpu_to_dump32(s, block_size);
@@ -926,7 +926,7 @@ static void create_header64(DumpState *s, Error **errp)
     size = sizeof(DiskDumpHeader64);
     dh = g_malloc0(size);
 
-    strncpy(dh->signature, KDUMP_SIGNATURE, strlen(KDUMP_SIGNATURE));
+    memcpy(dh->signature, KDUMP_SIGNATURE, SIG_LEN);
     dh->header_version = cpu_to_dump32(s, 6);
     block_size = s->dump_info.page_size;
     dh->block_size = cpu_to_dump32(s, block_size);