tools_common.h: fix usage_exit() prototype

+ the definitions in the examples

silences a missing-prototype warning

Change-Id: I7c064cacd6d2073a6107de6c3e2cc46ef8a68411

examples/vp8cx_set_ref.c

@@ -58,7 +58,7 @@
 
 static const char *exec_name;
 
-void usage_exit() {
+void usage_exit(void) {
   fprintf(stderr, "Usage: %s <width> <height> <infile> <outfile> <frame>\n",
           exec_name);
   exit(EXIT_FAILURE);