tools_common.h: fix usage_exit() prototype

+ the definitions in the examples

silences a missing-prototype warning

Change-Id: I7c064cacd6d2073a6107de6c3e2cc46ef8a68411

examples/decode_with_drops.c

@@ -65,7 +65,7 @@
 
 static const char *exec_name;
 
-void usage_exit() {
+void usage_exit(void) {
   fprintf(stderr, "Usage: %s <infile> <outfile> <N-M|N/M>\n", exec_name);
   exit(EXIT_FAILURE);
 }