examples/main.cpp: fix incorrect #include patch
examples/main.cpp: fix incorrect #include patch

When jsonbox is embedded in a larger project and the 'parent' CMakeLists.txt
points to the top-level jsonbox CMakeLists.tx file, the net result is that the
include directory for the examples/main.cpp file is different. Fix the #include
path to point to the Value.h file correctly.

spqr33/JsonBox,spqr33/JsonBox,anhero/JsonBox,anhero/JsonBox