Cleanup a compiler warning

This commit was SVN r13726.

opal/util/stacktrace.c

@@ -366,13 +366,12 @@ int opal_util_register_stackhandlers (void)
     char * string_value;
     char * tmp;
     char * next;
-    int param;
-    size_t i;
+    int param, i;
 
     gethostname(stacktrace_hostname, sizeof(stacktrace_hostname));
     stacktrace_hostname[sizeof(stacktrace_hostname) - 1] = '\0';
     /* to keep these somewhat readable, only print the machine name */
-    for (i = 0 ; i < sizeof(stacktrace_hostname) ; ++i) {
+    for (i = 0 ; i < (int)sizeof(stacktrace_hostname) ; ++i) {
         if (stacktrace_hostname[i] == '.') {
             stacktrace_hostname[i] = '\0';
             break;