m68k: math-emu: Add missing prototypes

When building with W=1:

arch/m68k/math-emu/fp_arith.c:301:16: warning: no previous prototype for ‘fp_fsglmul’ [-Wmissing-prototypes]
301 | struct fp_ext *fp_fsglmul(struct fp_ext *dest, struct fp_ext *src)
| ^~~~~~~~~~
arch/m68k/math-emu/fp_arith.c:357:16: warning: no previous prototype for ‘fp_fsgldiv’ [-Wmissing-prototypes]
357 | struct fp_ext *fp_fsgldiv(struct fp_ext *dest, struct fp_ext *src)
| ^~~~~~~~~~
CC arch/m68k/math-emu/fp_log.o
...

Fix this by adding the missing prototypes to header files.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230810141947.1236730-17-arnd@kernel.org/
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/96c039640d76973ea762d79018de0bc75bbdc1dc.1692283195.git.geert@linux-m68k.org

m68k: math-emu: Sanitize include guards

Some include guards start with an underscore, others don't.
Some comments do not match the actual include guard.

Make them uniform, adhering to the "FP_<FOO>_H" format.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/8c84c8bf06c0c1e463c47f071891e2e83d5abdd2.1692283195.git.geert@linux-m68k.org

Linux-2.6.12-rc2

Initial git repository build. I'm not bothering with the full history,
even though we have it. We can create a separate "historical" git
archive of that later if we want to, and in the meantime it's about
3.2GB when imported into git - space that would just make the early
git days unnecessarily complicated, when we don't have a lot of good
infrastructure for it.

Let it rip!