[CMAKE] Fix Miniupnpc error message

Summary:
The error message when the miniupnpc.h is not found is fubared due the
CMAKE_REQUIRED_INCLUDES not taking into account the NOTFOUND case. This
diff fixes the issue and restore the expected error.

Test Plan:
Uninstall miniupnpc, then

mkdir buildcmake && cd buildcmake
cmake -GNinja ..

Should output a CMake error:

Could NOT find MiniUPnPc (missing: MINIUPNPC_INCLUDE_DIR
MINIUPNPC_LIBRARY)

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3141