Fix compiling with 3D disabled due to unused navigation variable

[aaronfranke]

In the current master, Godot does not compile with 3D disabled scons target=template_debug disable_3d=yes, at least it fails with Clang, due to an unused variable. This PR fixes that.

[ 51%] In file included from modules/navigation/godot_navigation_server.cpp:31:
modules/navigation/godot_navigation_server.h:83:22: error: private field 'navmesh_generator_3d' is not used [-Werror,-Wunused-private-field]
        NavMeshGenerator3D *navmesh_generator_3d = nullptr;
                            ^
1 error generated.

[YuriSizov]

Thanks!