ARM: mvebu: add missing header

The definition for SIGBUS may not be visible without including
linux/signal.h, as I found during randconfig testing.

Adding an explicit include is certainly the right thing to do.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Acked-by: Gregory Clement <gregory.clement@free-electrons.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>