libxmlrpc: disable parallel build

Although upstream tries to support parallel builds, there are several
dependencies that, if fixed, would end up making the library build
linearly, so in the end there would be no benefit to fix those.

Fixes:

  http://autobuild.buildroot.net/results/996/996fc95b302fb6dfc7cd9a468fd395226a36c6c4/



Suggested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Alvaro G. M. <alvaro.gamez@hazent.com>
Signed-off-by: Alvaro G. M <alvaro.gamez@hazent.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>