cangen: mark locally used function static

sigterm isn't regularly called from outside the translation unit, thus
make it static.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>

cangen.c

@@ -128,7 +128,7 @@ void print_usage(char *prg)
 	fprintf(stderr, "(my favourite default :)\n\n");
 }
 
-void sigterm(int signo)
+static void sigterm(int signo)
 {
 	running = 0;
 }