selftests/bpf: Fix arena_spin_lock.c build dependency

Changing bpf_arena_spin_lock.h does not lead to recompiling
arena_spin_lock.c. By convention, all BPF progs depend on all
header files in progs/, so move this header file there. There
are no other users besides arena_spin_lock.c.

Signed-off-by: Ilya Leoshkevich <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: iii-i