Add call to rounding function in fixeddecimal tiny

[sffc]

For testing the impact of #4578

[sffc]

Main:

-rwxr-xr-x 1 root root    24248 Feb  5 23:27 optim4.elf
-rwxr-xr-x 1 root root    13520 Feb  5 23:27 optim5.elf

This branch:

-rwxr-xr-x 1 root root    25568 Feb  5 23:55 optim4.elf
-rwxr-xr-x 1 root root    14464 Feb  5 23:55 optim5.elf

Seems approximately reasonable; about 1 kB for the rounding function. Would like it to be smaller but at least it is not bigger.