Fix potential linker error and warning

[fmrico]

Hi,

Jenkins is failing at buildfarm:

/usr/bin/ld: libnavmap_ros.so: undefined reference to `navmap::LayerView<unsigned char>::content_hash() const'
/usr/bin/ld: libnavmap_ros.so: undefined reference to `navmap::LayerView<float>::content_hash() const'
/usr/bin/ld: libnavmap_ros.so: undefined reference to `navmap::LayerView<double>::content_hash() const'

This PR fixes this.