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.

Signed-off-by: Christian Beier <dontmind@freeshell.org>
Merge pull request #462 from bk138/gcc-workaround-sys-sdt-h-detection

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