[gdb/testsuite] Fix main in gdb.trace/mi-trace-frame-collected.exp

With test-case gdb.trace/mi-trace-frame-collected.exp I run into:
...
gdb compile failed, gdb.trace/actions.c: In function 'main':
gdb.trace/actions.c:139:1: warning: old-style function definition \
  [-Wold-style-definition]
  139 | main (argc, argv, envp)
      | ^~~~
...

Fix this by rewriting main into a prototyped function.

Tested on x86_64-linux.

PR testsuite/32756
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32756

gdb/testsuite/gdb.trace/actions.c

@@ -136,9 +136,7 @@ static void end ()	/* called after everything else */
 }
 
 int
-main (argc, argv, envp)
+main (int argc, char *argv[], char **envp)
-     int argc;
-     char *argv[], **envp;
 {
   int i;
   unsigned long myparms[10];