open-plc-utils: fix build on !uclibc toolchains

The open-plc-utils source code has a condition for uClibc toolchains
when calling vfprintf(). But the non-uClibc case simply doesn't
build. And the recommended solution at
https://github.com/qca/open-plc-utils/issues/36 is to pass
-D__UCLIBC__ unconditionally.

Fixes:

  http://autobuild.buildroot.org/results/4fd/4fdc56f5dec6c8773086a4661eff1e4ce6ef660e/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>