arm64: fix missing linux/bug.h include in asm/arch_timer.h

On next-20150105, defconfig compilation breaks with:

./arch/arm64/include/asm/arch_timer.h:112:2: error: implicit declaration of function ‘BUG’ [-Werror=implicit-function-declaration]

Fix by including linux/bug.h, where the BUG macro is defined.

This second version incorporates a comment from Mark Rutland
<mark.rutland@arm.com> to keep the includes in alphabetical order
by filename.
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Paul Walmsley <pwalmsley@nvidia.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/arch_timer.h

@@ -21,6 +21,7 @@
 
 #include <asm/barrier.h>
 
+#include <linux/bug.h>
 #include <linux/init.h>
 #include <linux/types.h>