TimePrint: minor fixup for libfmt 10

libfmt version 10 has difficulties formatting a `StringBuffer`, and we
need to help it by explicitly invoking the `c_str()` method.

NEWS

@@ -10,6 +10,7 @@ ver 0.23.13 (not yet released)
 * Linux
   - shut down if parent process dies in --no-daemon mode
   - determine systemd unit directories via pkg-config
+* support libfmt 10
 
 ver 0.23.12 (2023/01/17)
 * input

src/TimePrint.cxx

@@ -36,5 +36,5 @@ time_print(Response &r, const char *name,
 		return;
 	}
 
-	r.Fmt(FMT_STRING("{}: {}\n"), name, s);
+	r.Fmt(FMT_STRING("{}: {}\n"), name, s.c_str());
 }