gcc: work around wrongly detected sys/sdt.h header.

The build system somehow detects /usr/include/sys/sdt.h, which is part of systemtap-sdt-devel
package in the host environment but not present in the SDK.

This patch simply disables the check for sys/sdt.h, enabling building of this package when
systemtap-sdt-devel is installed in the host environment.

Fixes https://github.com/openwrt/packages/issues/296

Signed-off-by: Christian Beier <dontmind@freeshell.org>