Make check-installed-headers.sh ignore sys/sysctl.h for x32.

check-installed-headers tests were failing for x32 because of the x86
bits/sysctl.h containing a #error for x32.  This patch makes the tests
ignore sys/sysctl.h for x32, similar to the other special-casing of
particular headers.

Tested for x86_64 (full testing for -m64, compile-only for x32).

	* scripts/check-installed-headers.sh: Ignore sys/sysctl.h for x32.