selftests/seccomp: Fix seccomp failure by adding missing headers

seccomp_bpf failed on tests 47 global.user_notification_filter_empty
and 48 global.user_notification_filter_empty_threaded when it's
tested on updated kernel but with old kernel headers. Because old
kernel headers don't have definition of macro __NR_clone3 which is
required for these two tests. Since under selftests/, we can install
headers once for all tests (the default INSTALL_HDR_PATH is
usr/include), fix it by adding usr/include to the list of directories
to be searched. Use "-isystem" to indicate it's a system directory as
the real kernel headers directories are.

Signed-off-by: Sherry Yang <[e-mail]>
Tested-by: Sherry Yang <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/seccomp/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
-CFLAGS += -Wl,-no-as-needed -Wall
+CFLAGS += -Wl,-no-as-needed -Wall -isystem ../../../../usr/include/
 LDFLAGS += -lpthread
 
 TEST_GEN_PROGS := seccomp_bpf seccomp_benchmark