selftests: Correct the headers install path

uapi headers should be installed at the top of the object tree,
"<obj_tree>/usr/include". There is no need for kernel headers to
be present at kselftest build directory, "<obj_tree>/kselftest/usr/
include" as well. This duplication can be avoided by correctly
specifying the INSTALL_HDR_PATH.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>