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 <james.hilliard1@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/bpf/20220826035141.737919-1-james.hilliard1@gmail.com

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

@@ -19,7 +19,7 @@ struct {
 int count = 0;
 int noise = 0;
 
-__always_inline int subprog_noise(void)
+static __always_inline int subprog_noise(void)
 {
 	__u32 key = 0;