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: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>