Do not include asm/cachectl.h in nios2 sys/cachectl.h.

The nios2 sys/cachectl.h includes a kernel header asm/cachectl.h,
which does not exist, so causing the check-installed-headers tests to
fail.  This patch removes the include of a nonexistent header.

Tested (compilation only) for nios2.

	* sysdeps/unix/sysv/linux/nios2/sys/cachectl.h: Do not include
	<asm/cachectl.h>.