Fix incomplete declarations to squelch warnings

[JeffreyBroll]

There are quite a few warnings of the form
passing arguments to a function without a prototype is deprecated in all versions of C and is not supported in C2x [-Wdeprecated-non-prototype]
that we can silence now and prevent from graduating into errors with future C.

These seem to be caused by

• incompletely specifying the Lgm_MagModelInfo's Bfield member, a pointer to a function that we can prototype fully if we first declare the struct (since *Bfield takes two vecs and a magmodelinfo as arguments), and
• some similar things in libLanlGeoMag/Lgm_B_FromScatteredData.c where we just do the same to remove fussing there

This tests with our CrayPE, my local clang, and my local gcc (although the last one fusses with newer GNU and C standards).