test: unpack_properties: add constructor hint
test: unpack_properties: add constructor hint

scan-build-14 with GCC-12 seems to be failing with the following
warning:
```
../../../test/unpack_properties.cpp:34:23: error: no matching member function for call to 'emplace'
                error.emplace(reason, property);
```

Add a constructor to provide a hint and make scan-build happier.

Signed-off-by: Patrick Williams <cbb7353e6d953ef360baf960c122346276c6e320@stwcx.xyz>
Change-Id: Ie5fa169a9c86e8c221adb5407efb6bef5681c9e0

openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus