conflict with previous declaration of 'putchar'

Unable to build with sdcc 3.9.0:

```
CC radio/golay.c
CC radio/serial.c
radio/serial.c:541: error 98: conflict with previous declaration of 'putchar' for attribute 'type' at /usr/bin/../share/sdcc/include/stdio.h:86
from type 'int function ( int fixed) fixed'
to type 'void function ( unsigned-char fixed) __reentrant fixed'
make[1]: *** [include/rules.mk:135: obj/rfd900a/radio~rfd900a/serial.rel] Error 1
make[1]: Leaving directory '/home/paulk/botlink/SiK/Firmware'
make: *** [Makefile:95: install~radio~rfd900a] Error 2
```

I changed the signature in serial.c to
```
int
putchar(int c) __reentrant
```

and it's working fine for me. But I suspect there's a more correct fix.

I found [what looks like the same issue](https://sourceforge.net/p/sdcc/mailman/message/12702250/) on the sdcc mailing list from 2007, but the PUTCHAR definition doesn't exist, so the reply there was unhelpful.

Start in radio/serial.c around line 541 and compare its putchar declaration with the declaration in the SDCC stdio.h path shown by the compiler. Reproduce the install~radio~rfd900a build with SDCC 3.9.0, then confirm the corrected declaration removes the conflict and the target builds successfully.

Written by the indexing model from the issue text.