Remove libattr requirement

RHEL/CentOS 6 supports sys/xattr.h eliminating the need for
libattr-devel as a dependency.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: DHE <git@dehacked.net>
Closes #7344
Closes #7351

tests/zfs-tests/cmd/xattrtest/xattrtest.c

@@ -37,7 +37,7 @@
 #include <fcntl.h>
 #include <time.h>
 #include <unistd.h>
-#include <attr/xattr.h>
+#include <sys/xattr.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <sys/stat.h>