Can't build loader as static library

Attempting to build the loader with `-DDYNAMIC_LOADER=OFF` results in the following cmake error:
```
CMake Error: install(EXPORT "openxr_loader_export" ...) includes target "openxr_loader" which requires target "jsoncpp_interface" that is not in any export set.
CMake Error in src/loader/CMakeLists.txt:
export called with target "openxr_loader" which requires target
"jsoncpp_interface" that is not in any export set.
```

Start in src/loader/CMakeLists.txt and reproduce the configuration with -DDYNAMIC_LOADER=OFF. Trace the openxr_loader and jsoncpp_interface export declarations; done means the static-loader configuration completes without the reported CMake export error.

Written by the indexing model from the issue text.