selftests/bpf: rename btf_decl_tag.c to test_btf_decl_tag.c

The uapi btf.h contains the following declaration:
struct btf_decl_tag {
__s32 component_idx;
};

The skeleton will also generate a struct with name
"btf_decl_tag" for bpf program btf_decl_tag.c.

Rename btf_decl_tag.c to test_btf_decl_tag.c so
the corresponding skeleton struct name becomes
"test_btf_decl_tag". This way, we could include
uapi btf.h in prog_tests/btf_tag.c.
There is no functionality change for this patch.

Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220127154611.656699-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>