selftests: use "$(MAKE)" instead of "make" for headers_install

If top make invocation uses -j4 or larger, this patch reduces
"make headers_install" subtask run time from 30 to 7 seconds.

CC: Shuah Khan <shuah@kernel.org>
CC: Shuah Khan <skhan@linuxfoundation.org>
CC: linux-kselftest@vger.kernel.org
CC: linux-kernel@vger.kernel.org
Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Author: dvlasenk

tools/testing/selftests/lib.mk

@@ -47,9 +47,9 @@ ARCH		?= $(SUBARCH)
 khdr:
 ifndef KSFT_KHDR_INSTALL_DONE
 ifeq (1,$(DEFAULT_INSTALL_HDR_PATH))
-	make --no-builtin-rules ARCH=$(ARCH) -C $(top_srcdir) headers_install
+	$(MAKE) --no-builtin-rules ARCH=$(ARCH) -C $(top_srcdir) headers_install
 else
-	make --no-builtin-rules INSTALL_HDR_PATH=$$OUTPUT/usr \
+	$(MAKE) --no-builtin-rules INSTALL_HDR_PATH=$$OUTPUT/usr \
 		ARCH=$(ARCH) -C $(top_srcdir) headers_install
 endif
 endif