[C++] pkg-config flags don't include -ldl for static builds

[bkietz]

pkg-config's libraries are not automatically derived from /ARROW_(STATIC|SHARED)_INSTALL_INTERFACE_LIBS/, instead they are managed in independent variables /ARROW_PC_LIBS(|_PRIVATE). When linking statically, pkg-config doesn't pick up the new dependency on libdl introduced by #39067

This produces unresolved symbol errors

[bkietz]

Issue resolved by pull request 40578
#40578