arm64/hw_breakpoint.c: remove unnecessary header

Header <asm/kdebug.h> is not needed for arm64/hw_breakpoint.c,
Removing the same.

Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
Reviewed-by: Vaneet Narang <v.narang@samsung.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>