software/demo: Fix some warnings.

litex/soc/software/demo/donut.c

@@ -21,6 +21,7 @@
 
 signed char b[1760], z[1760];
 
+void donut(void);
 void donut(void) {
   int sA=1024,cA=0,sB=1024,cB=0,_;
   for (;;) {

litex/soc/software/demo/helloc.c

@@ -1,5 +1,6 @@
 #include <stdio.h>
 
+void helloc(void);
 void helloc(void) {
   printf("C: Hello, world!\n");
 }