Scalar.h and Scalar.hpp have the same header guard in tf2/LinearMath

[philippewarren]

Since #720, .h files where moved to equivalent .hpp files, and the tf2/LinearMath/Scalar files have the same header guard.
Since the only job of the .h now is to include the .hpp, using the .h directly never includes anything because of the shared header guard.

What should be a deprecation warning is instead a bunch of missing stuff creating a whole lot of errors.

This should be fixed to ease the migration to .hpp.

(Using 0.25.11 on Humble)

[ahcorde]

PR is merged

[ahcorde]

#755