selftests: Add and export a kernel uapi headers path

Kernel uapi headers can be present at different paths depending upon
how the build was invoked. It becomes impossible for the tests to
include the correct headers directory. Set and export KHDR_INCLUDES
variable to make it possible for sub make files to include the header
files.
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/Makefile

@@ -129,8 +129,11 @@ ifneq ($(KBUILD_OUTPUT),)
   # $(realpath ...) resolves symlinks
   abs_objtree := $(realpath $(abs_objtree))
   BUILD := $(abs_objtree)/kselftest
+  KHDR_INCLUDES := -I${abs_objtree}/usr/include
 else
   BUILD := $(CURDIR)
+  abs_srctree := $(shell cd $(top_srcdir) && pwd)
+  KHDR_INCLUDES := -I${abs_srctree}/usr/include
   DEFAULT_INSTALL_HDR_PATH := 1
 endif
 
@@ -139,6 +142,7 @@ include $(top_srcdir)/scripts/subarch.include
 ARCH           ?= $(SUBARCH)
 export KSFT_KHDR_INSTALL_DONE := 1
 export BUILD
+export KHDR_INCLUDES
 
 # set default goal to all, so make without a target runs all, even when
 # all isn't the first target in the file.