Why do we need to include xattr.h twice? Removed attr/xattr.h

I tried to build on CentOS 6. I noticed that there two issues with a new install.

configure.ac is looking for json-c in /usr/local/include/json-c. In you install json-c-devel the directory is /usr/include/json. 

backend.c in libdroplet/posix includes xattr.h twice, once from linux and once from attr (under /usr/include). I can not file a file attr/xattr.h owned by a package. I believe we only need to include 1 version of xattr.h and the one in /usr/include/linux appears to work.