selftests/bpf: Declare subprog_noise as static in tailcall_bpf2bpf4

Due to bpf_map_lookup_elem being declared static we need to also
declare subprog_noise as static.

Fixes the following error:
progs/tailcall_bpf2bpf4.c:26:9: error: 'bpf_map_lookup_elem' is static but used in inline function 'subprog_noise' which is not static [-Werror]
26 | bpf_map_lookup_elem(&nop_table, &key);
| ^~~~~~~~~~~~~~~~~~~

Signed-off-by: James Hilliard <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Song Liu <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c

···
 int count = 0;
 int noise = 0;
 
-__always_inline int subprog_noise(void)
+static __always_inline int subprog_noise(void)
 {
 	__u32 key = 0;