add ../../lib to rpath for vendor fuzz targets

For "normal" fuzz targets, the binary goes in
/data/fuzz/<arch>/<fuzzer_name>/fuzzer_name>
with libraries in
/data/fuzz/<arch>/lib/

If 'vendor: true' is set, the fuzz target binary get placed in
/data/fuzz/<arch>/<fuzzer_name>/vendor/<fuzzer_name>.

We need an extra '..' in the rpath to link to the included libraries
correctly.

Bug: 254723623
Test: built libsrtp2-fuzzer, checked linking with ldd
Change-Id: I24282d7b252142ed795a6091fece2d56125329dc