scripts: create CT_HEADERS_DIR

"${CT_SYSROOT_DIR}/usr/include" is only for "${CT_USE_SYSROOT}" = "y".
We should also mkdir when "${CT_USE_SYSROOT}" != "y".
"${CT_HEADERS_DIR}" can support both cases.

Signed-off-by: Zhenqiang Chen <zhenqiang.chen@linaro.org>
(transplanted from bdff4164a1227ebdd538e0ad3c9b6862b3c5a253)

 #!/bin/sh

 tail -n+5 "${0}" |bzcat

 exit 0