dumper: Fix introspection binaries missing rpaths for relative dylibs

Hi all,

Apple expects dylibs to have an ID of the form @rpath/libyadda.dylib 1. For this convention to work, a RPATH entry pointing to the library prefix and/or to the relative location of said prefix w.r.t the dependent executable or library, must be inserted.

This need previously did not arose because gobject-introspection implicitly relied on the consumers having IDs set to their absolute paths. This is no longer the case with GStreamer's Cerbero, which sanitizes dylib IDs at install time.

The simplest and most obvious fix is to ensure that, for each library load path, the corresponding rpath entry is inserted.

Let me know what you think.