package/openvmtools: fix procps-ng library name

procps-ng libraries are now accesible on the same name as procps, thus
using the CUSTOM_PROCPS_NAME environment variable breaks compilation.

Fixes:

  http://autobuild.buildroot.net/results/2a1/2a19f84c35de0ff008c2cd1aea96f6a4f2767ee1/



Signed-off-by: Karoly Kasza <kaszak@gmail.com>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Tested-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>