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

 # Needed for systems without gettext

 $* -xc -o /dev/null - > /dev/null 2>&1 << EOF

 #include <libintl.h>

 int main()

 {

 	gettext("");

 	return 0;

 }

 EOF

 if [ ! "$?" -eq "0"  ]; then

 	echo -DKBUILD_NO_NLS;

 fi