Describe your environment
When "WITH_ZIPKIN" option is ON while compile Zipkin example, compiler complains about that "opentelemetry/exporters/zipkin/zipkin_exporter_factory.h" file not found.

Steps to reproduce
set(WITH_EXAMPLES ON)
set(WITH_ZIPKIN ON)

What is the expected behavior?
Zipkin example should be compiled successfully when its example is enabled.

What is the actual behavior?
Compiler error due to not found header file.

Additional context
Zipkin folder inside of "exporters" folder there is a CMakefile of this component. In this CMakefile, target_include_directories was not defined. In my opinion, this is a problem that needs to be fixed.