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 <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20250424165525.154403-2-iii@linux.ibm.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>