selftest/memfd: include default header install path

Include the default path for INSTALL_HDR_PATH to make it less intrusive when
cross building.
Signed-off-by: Tyler Baker <tyler.baker@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/memfd/Makefile

@@ -2,6 +2,7 @@ CC = $(CROSS_COMPILE)gcc
 CFLAGS += -D_FILE_OFFSET_BITS=64
 CFLAGS += -I../../../../include/uapi/
 CFLAGS += -I../../../../include/
+CFLAGS += -I../../../../usr/include/
 
 all:
 	$(CC) $(CFLAGS) memfd_test.c -o memfd_test