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>

tools/testing/selftests/Makefile

@@ -167,7 +167,7 @@ khdr:
 ifeq (1,$(DEFAULT_INSTALL_HDR_PATH))
 	$(MAKE) --no-builtin-rules ARCH=$(ARCH) -C $(top_srcdir) headers_install
 else
-	$(MAKE) --no-builtin-rules INSTALL_HDR_PATH=$$BUILD/usr \
+	$(MAKE) --no-builtin-rules INSTALL_HDR_PATH=$(abs_objtree)/usr \
 		ARCH=$(ARCH) -C $(top_srcdir) headers_install
 endif