bpf: Avoid __hidden__ attribute in static object

An object defined as `static' defaults to hidden visibility.  If
additionally the visibility(__weak__) compiler attribute is applied to
the declaration of the object, GCC warns that the attribute gets
ignored.

This patch removes the only instance of this problem among the BPF
selftests.

Tested in bpf-next master.
Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20240507074227.4523-2-jose.marchesi@oracle.com

tools/testing/selftests/bpf/progs/cpumask_common.h

@@ -9,7 +9,7 @@
 
 int err;
 
-#define private(name) SEC(".bss." #name) __hidden __attribute__((aligned(8)))
+#define private(name) SEC(".bss." #name) __attribute__((aligned(8)))
 private(MASK) static struct bpf_cpumask __kptr * global_mask;
 
 struct __cpumask_map_value {