selftests: use -isystem instead of -I to include headers

selftests: use -isystem instead of -I to include headers

Selftests need kernel headers and glibc for compilation.  In compilation
of selftests, uapi headers from kernel source are used instead of default
ones while glibc has already been compiled with different header files
installed in the operating system.  So there can be redefinition warnings
from compiler.  These warnings can be suppressed by using -isystem to
include the uapi headers.

Link: https://lkml.kernel.org/r/20220214160756.3543590-1-usama.anjum@collabora.com
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>