arrow

GH-40577: [C++] Ensure pkg-config flags include -ldl for static builds (#40578)



### Rationale for this change

When linking statically, pkg-config doesn't pick up the new dependency on libdl introduced by https://github.com/apache/arrow/pull/39067

This produces [unresolved symbol errors](https://github.com/apache/arrow/pull/39067#issuecomment-1999218559)

### What changes are included in this PR?

Addition of `-ldl` to `ARROW_PC_LIBS_PRIVATE` to ensure linkage to the necessary library

### Are these changes tested?

yes

### Are there any user-facing changes?

no

* GitHub Issue: #40577

Authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>