Fix install interface to point to correct include dir

The contents of the include directory are actually installed to
$PREFIX/include/sparta. So it is actually $PREFIX/include/sparta that
needs to be provided as an include directory, not just $PREFIX/include.

CMakeLists.txt

@@ -28,7 +28,7 @@ add_library(sparta INTERFACE)
 
 target_include_directories(sparta INTERFACE
       $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
-      $<INSTALL_INTERFACE:include>
+      $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}/sparta>
       )
 
 target_link_libraries(sparta INTERFACE ${Boost_LIBRARIES})