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