[cmake] Linking MathLib to MKL privately is sufficient.

MathLib/CMakeLists.txt

@@ -32,9 +32,9 @@ target_link_libraries(
            $<$<BOOL:${OGS_USE_LIS}>:${LIS_LIBRARIES}>
            $<$<BOOL:${OGS_USE_CVODE}>:CVODE::CVODE>
            $<$<BOOL:${OGS_USE_PETSC}>:PkgConfig::PETSC>
-           $<$<BOOL:${OGS_USE_MKL}>:MKL::MKL>
            Eigen3::Eigen
            $<$<TARGET_EXISTS:OpenMP::OpenMP_CXX>:OpenMP::OpenMP_CXX>
+    PRIVATE $<$<BOOL:${OGS_USE_MKL}>:MKL::MKL>
 )
 if(OGS_USE_MKL AND "${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
     # Otherwise required MKL libs get stripped out (don't show up in ldd):