mpc: optimize HZ retrieval

As Brian De Wolf noted:

hz only really needs to be calculated once, so this line could be
moved up out of the loop, resulting in a likely unnoticeable
performance increase. :)

tools/munin-plugins-busybox/cpu.c

@@ -136,8 +136,8 @@ int cpu(int argc, char **argv) {
 	}
 	if(!(f=fopen(PROC_STAT, "r")))
 		return fail("cannot open " PROC_STAT);
+	hz = getenvint("HZ", 100);
 	while(fgets(buff, 256, f)) {
-		hz = getenvint("HZ", 100);
 		if(!strncmp(buff, "cpu ", 4)) {
 			fclose(f);
 			if(!(s = strtok(buff+4, " \t")))