package/mono: fixing multiple definition of vasprintf

This patch fixes the detection of an available
vasprintf function, this way the internal vasprintf
function is not compiled.

Fixes http://autobuild.buildroot.net/results/3ab2b8667e5657bf07f55dd04dd857167ab4c62a/
      http://autobuild.buildroot.net/results/3ab2b8667e5657bf07f55dd04dd857167ab4c62a/



Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>