fix link error for write_byte using gcc 7.2

[zabereer]

gcc -Wall -DTEST lib/blinkt.c -l bcm2835 -o blinkt /tmp/ccjHD5qV.o: In functionshow': blinkt.c:(.text+0x318): undefined reference to write_byte' blinkt.c:(.text+0x320): undefined reference towrite_byte' blinkt.c:(.text+0x328): undefined reference to write_byte' blinkt.c:(.text+0x330): undefined reference towrite_byte' blinkt.c:(.text+0x374): undefined reference to write_byte' /tmp/ccjHD5qV.o:blinkt.c:(.text+0x39c): more undefined references towrite_byte' follow collect2: error: ld returned 1 exit status make: *** [Makefile:2: all] Error 1 `

[Gadgetoid]

Thank you- I'd pretty much abandoned the C code in favour of a pure-Python approach, but if you're getting some use out of it there's no reason not to keep it in good shape.