tests: Add POPT_CFLAGS to gen-ust-events-ns and gen-ns-events

tests: Add POPT_CFLAGS to gen-ust-events-ns and gen-ns-events

Observed issue
==============

When building in an environment where popt was in a non-standard
location, the builds for these two test binaries failed with the
following error:

```
gen-ns-events.cpp:19:10: fatal error: popt.h: No such file or directory
   19 | #include <popt.h>
      |          ^~~~~~~~
```

Solution
========

Set the binary-specific CPPFLAGS in the `Makefile.am`

Known drawbacks
===============

None

Change-Id: I5563e24f330be86d630c68c32eaafaedf53a6c87
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>