cl-clang: test_serialization: class boost::archive::archive_exception: unregistered class - derived class not registered or exported

New issue uncovered by adding a cl-clang Appveyor build job:

https://ci.appveyor.com/project/jeking3/ptr-container-lviqw/builds/25773256/job/uvt8w5fjj452kjw4#L1554
```
testing.capture-output bin.v2\libs\ptr_container\test\serialization.test\clang-win-8.0.0\release\address-model-64\cxxstd-11-iso\threading-multi\serialization.run
====== BEGIN OUTPUT ======
Running 1 test case...
unknown location(0): fatal error: in "Pointer Container Test Suite/test_serialization": class boost::archive::archive_exception: unregistered class - derived class not registered or exported
*** 1 failure is detected in the test module "Master Test Suite"

EXIT STATUS: 201
====== END OUTPUT ======
set Path=C:\projects\boost-root\bin.v2\libs\serialization\build\clang-win-8.0.0\release\address-model-64\cxxstd-11-iso\threading-multi;%Path%
set status=0
if %status% NEQ 0 (
echo Skipping test execution due to testing.execute=off
exit 0
)
"bin.v2\libs\ptr_container\test\serialization.test\clang-win-8.0.0\release\address-model-64\cxxstd-11-iso\threading-multi\serialization.exe" > "bin.v2\libs\ptr_container\test\serialization.test\clang-win-8.0.0\release\address-model-64\cxxstd-11-iso\threading-multi\serialization.output" 2>&1
set status=%ERRORLEVEL%
echo. >> "bin.v2\libs\ptr_container\test\serialization.test\clang-win-8.0.0\release\address-model-64\cxxstd-11-iso\threading-multi\serialization.output"
echo EXIT STATUS: %status% >> "bin.v2\libs\ptr_container\test\serialization.test\clang-win-8.0.0\release\address-model-64\cxxstd-11-iso\threading-multi\serialization.output"
if %status% EQU 0 (
copy "bin.v2\libs\ptr_container\test\serialization.test\clang-win-8.0.0\release\address-model-64\cxxstd-11-iso\threading-multi\serialization.output" "bin.v2\libs\ptr_container\test\serialization.test\clang-win-8.0.0\release\address-model-64\cxxstd-11-iso\threading-multi\serialization.run"
)
set verbose=0
if %status% NEQ 0 (
set verbose=1
)
if %verbose% EQU 1 (
echo ====== BEGIN OUTPUT ======
type "bin.v2\libs\ptr_container\test\serialization.test\clang-win-8.0.0\release\address-model-64\cxxstd-11-iso\threading-multi\serialization.output"
echo ====== END OUTPUT ======
)
exit %status%
...failed testing.capture-output bin.v2\libs\ptr_container\test\serialization.test\clang-win-8.0.0\release\address-model-64\cxxstd-11-iso\threading-multi\serialization.run...
```