libbpf: Fix missing #pragma in libbpf_utils.c

The recent sha256 patch uses a GCC pragma to suppress compile errors for
a packed struct, but omits a needed pragma (see related link) and thus
still raises errors: (e.g. on GCC 12.3 armhf)

libbpf_utils.c:153:29: error: packed attribute causes inefficient alignment for ‘__val’ [-Werror=attributes]
  153 | struct __packed_u32 { __u32 __val; } __attribute__((packed));
      |                             ^~~~~

Resolve by adding the GCC diagnostic pragma to ignore "-Wattributes".

Link: https://lore.kernel.org/bpf/CAP-5=fXURWoZu2j6Y8xQy23i7=DfgThq3WC1RkGFBx-4moQKYQ@mail.gmail.com/



Fixes: 4a1c9e54 ("libbpf: remove linux/unaligned.h dependency for libbpf_sha256()")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>