trace/simple: Fix warning and wrong trace file name for MinGW

On Windows, getpid() always returns an int value, but pid_t (which is
expected by the format string) is either a 32 bit or a 64 bit value.

Without a type cast (or a modified format string), the compiler prints
a warning when building for 64 bit Windows and the resulting trace_file_name
will include a wrong pid:

trace/simple.c:332:9: warning:
 format ‘%lld’ expects argument of type ‘long long int’,
 but argument 2 has type ‘int’ [-Wformat=]

Signed-off-by: Stefan Weil <sw@weilnetz.de>

trace/simple.c

@@ -329,7 +329,8 @@ bool st_set_trace_file(const char *file)
     g_free(trace_file_name);
 
     if (!file) {
-        trace_file_name = g_strdup_printf(CONFIG_TRACE_FILE, getpid());
+        /* Type cast needed for Windows where getpid() returns an int. */
+        trace_file_name = g_strdup_printf(CONFIG_TRACE_FILE, (pid_t)getpid());
     } else {
         trace_file_name = g_strdup_printf("%s", file);
     }