Makefile: Remove dangerous EOL trailing backslash

Makefile: Remove dangerous EOL trailing backslash

One might get caught trying to understand unexpected Makefile
behavior. Trailing backslash can help to split very long lines,
but are rather dangerous when nothing follow. Preserve other
developers debugging time by removing this one.

Reviewed-by: Thomas Huth <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>