polkit-agent: Don't print a warning if not installed

We don't want to have a hard dependency on polkit.  Some read-only DBus
APIs (e.g. `rpm-ostree status`) can be invoked unprivileged.

xref https://bugzilla.redhat.com/show_bug.cgi?id=2105405

src/app/rpmostree-polkit-agent.cxx

@@ -33,6 +33,7 @@
 #include <string.h>
 #include <sys/poll.h>
 #include <sys/prctl.h>
+#include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>
@@ -215,6 +216,17 @@ rpmostree_polkit_agent_open (void)
   if (!isatty (STDIN_FILENO))
     return 0;
 
+  /* Don't print an error if there's no pkttyagent; we don't want
+   * to have a hard dependency on it.  Not all DBus methods require
+   * it.
+   */
+  struct stat stbuf;
+  if (stat (POLKIT_TTY_AGENT_BINARY_PATH, &stbuf) < 0)
+    {
+      if (errno == ENOENT)
+        return 0;
+    }
+
   if (pipe (pipe_fd) < 0)
     return -errno;