bpftool: Fix -Wuninitialized-const-pointer warnings with clang >= 21

This fixes the build with -Werror -Wall.

btf_dumper.c:71:31: error: variable 'finfo' is uninitialized when passed as a const pointer argument here [-Werror,-Wuninitialized-const-pointer]
   71 |         info.func_info = ptr_to_u64(&finfo);
      |                                      ^~~~~

prog.c:2294:31: error: variable 'func_info' is uninitialized when passed as a const pointer argument here [-Werror,-Wuninitialized-const-pointer]
 2294 |         info.func_info = ptr_to_u64(&func_info);
      |

v2:
  - Initialize instead of using memset.

Signed-off-by: Tom Stellard <tstellar@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/bpf/20250917183847.318163-1-tstellar@redhat.com