Fix function relocations in libzpool

binutils 2.23.1 fails in situations that generate function relocations
on PowerPC and possibly other architectures. This causes linking of
libzpool to fail because it depends on libnvpair. We add a dependency on
libnvpair to lib/libzpool/Makefile.am to correct that.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>