Always use QJSON_LIBRARIES

QJson specifies this since 2 years in its Config.cmake
QJSON_LDFLAGS was only needed when pkg-config was used.
Add QJson as a dependency in pkg-config
Merge pull request #14 from xhochy/qjson-libraries

Always use QJSON_LIBRARIES
Merge pull request #16 from xhochy/qjson-pc

Add QJson as a dependency in pkg-config