selftests: be sure to make khdr before other targets

LKP/0Day reported some building errors about kvm, and errors message
are not always same:
- lib/x86_64/processor.c:1083:31: error: ‘KVM_CAP_NESTED_STATE’ undeclared
(first use in this function); did you mean ‘KVM_CAP_PIT_STATE2’?
- lib/test_util.c:189:30: error: ‘MAP_HUGE_16KB’ undeclared (first use
in this function); did you mean ‘MAP_HUGE_16GB’?

Although kvm relies on the khdr, they still be built in parallel when -j
is specified. In this case, it will cause compiling errors.

Here we mark target khdr as NOTPARALLEL to make it be always built
first.

CC: Philip Li <philip.li@intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/lib.mk

@@ -48,6 +48,7 @@ ARCH		?= $(SUBARCH)
 # When local build is done, headers are installed in the default
 # INSTALL_HDR_PATH usr/include.
 .PHONY: khdr
+.NOTPARALLEL:
 khdr:
 ifndef KSFT_KHDR_INSTALL_DONE
 ifeq (1,$(DEFAULT_INSTALL_HDR_PATH))