CMake: limit visibility of COLVARS lepton library headers

cmake/CMakeLists.txt

@@ -402,7 +402,8 @@ foreach(SIMPLE_LIB REAX MEAM POEMS USER-ATC USER-AWPMD USER-COLVARS USER-H5MD
       target_include_directories(h5md PUBLIC ${LAMMPS_LIB_SOURCE_DIR}/h5md/include)
     elseif(PKG_LIB STREQUAL colvars)
       target_compile_options(colvars PRIVATE -DLEPTON)
-      target_include_directories(colvars PUBLIC ${LAMMPS_LIB_SOURCE_DIR}/colvars/lepton/include ${LAMMPS_LIB_SOURCE_DIR}/colvars)
+      target_include_directories(colvars PRIVATE ${LAMMPS_LIB_SOURCE_DIR}/colvars/lepton/include)
+      target_include_directories(colvars PUBLIC ${LAMMPS_LIB_SOURCE_DIR}/colvars)
     else()
       target_include_directories(${PKG_LIB} PUBLIC ${LAMMPS_LIB_SOURCE_DIR}/${PKG_LIB})
     endif()