To add rpath support (helps with ubuntu18 new rpath behavior) 

Hi,

in order to correctly "libmatlogger2-backend-matio.so" library, @alaurenzi suggested to add the rpath to CMakeLists.txt:
From:
`target_link_libraries(${LIBRARY_TARGET_NAME} PRIVATE dl -pthread)
`
To:
`target_link_libraries(${LIBRARY_TARGET_NAME} PRIVATE dl -pthread -Wl,-rpath,$ORIGIN)
`

Thank you for your help!