ltp-testsuite: don't include host headers

ltp-testsuite: don't include host headers

$SYSROOT defaults to empty, so we must set it to our sysroot.

Fixes:
http://autobuild.buildroot.net/results/812/812986b00786972d4e7fbb0362460a0de8a70472/
http://autobuild.buildroot.net/results/4a3/4a3c7ea4eb08248f2aec45d8efc111087bc1327f/
http://autobuild.buildroot.net/results/8b6/8b6f7adb5546387c25f3a4c9d8e6f72f4b62b9a4/

and more.

Cc: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>