samples/seccomp: Fix 32-bit build

Both the .o and the actual executable need to be built with -m32 in order
to link correctly.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: James Morris <jmorris@namei.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: fec7b669

 ("samples: add an example of seccomp user trap")
Link: http://lkml.kernel.org/r/20190107231631.1849-1-tycho@tycho.ws
Signed-off-by: Ingo Molnar <mingo@kernel.org>