postgresql: disable rpath

The linker -rpath option causes the linker to search for libraries in host
directories.

Should fix:
http://autobuild.buildroot.net/results/2cc/2cca5f8d8a8cd1bf9bbbc8c955aec477839e059f/
http://autobuild.buildroot.net/results/da6/da61d30791f216033971d679c84704d0fc0c3de0/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>