Fix compiler warnings in grdppa.c

[remkos]

Because I got a lot of warnings in compiling grdppa.c
Biggest problem was the missing #include gmt_internals.h

[PaulWessel]

So far, only gmt_*.c library files include gmt_internals.h and none of the modules do. What does grdppa need that is not accessible via gmt_prototypes.h (which is included in gmt_dev.h for module use)? TO keep this separation up we sometimes promote functions that are in gmt_internals.h to gmt_prototypes.h so that we don't need to add one-time EXTERM_MSC lines in the modules. As I write this, gmt_prototypes.h is not a great self-intuitive name for what it means...

[joa-quim]

Hmm, but I don't need to #include "gmt_internals.h" and neither needs CI because the PRs passed the build step.

[remkos]

It does compile without #include gmt_internals.h, but then there is still a (reasonable) warning:

[ 70%] Building C object src/CMakeFiles/supplib.dir/misc/grdppa.c.o
src/misc/grdppa.c:503:2: warning: implicit declaration of function 'gmtlib_grd_flip_vertical' is invalid
      in C99 [-Wimplicit-function-declaration]
        gmtlib_grd_flip_vertical (G->data, G->header->mx, G->header->my, 0, sizeof(G->data[0]));
        ^
1 warning generated.

Obviously, gmtlib_grd_flip_vertical needs a prototype. I'll add it to gmt_prototypes.h then.

[remkos]

In fact, no ... if you use a gmtlib routine, there is no way avoiding #include gmt_internals.h or EXTERN_MSC.

[remkos]

So EXTERN_MSC added. But I don't like it. I prefer we use gmt_internals.h whenever we need any gmtlib routine.

[PaulWessel]

I guess this depends on whether there is any benefit to us to keep functions only used within the GMT library as a separate include file (gmt_internals.h). If we were to include gmt_internals.h in gmt_dev.h, why even have separate gmt_prototypes.h and gmt_internals.h files? Isn't it better to rename any gmtlib_* function to gmt_* and promote to gmt_prototypes.h instead?

[joa-quim]

Yes, promoting it to gmt_* would be better. It's not rare that I need to do this operation to grids (mem layouts and stuff like that).