os: Round fraction in pnprintf %f format

Truncating the fraction part leads to a test failure where -1203.30 is
printed as -1203.29. Round this to the nearest value instead by adding
0.5 before converting to an integer

Signed-off-by: Keith Packard <keithp@keithp.com>

os/utils.c

@@ -1999,7 +1999,7 @@ FormatDouble(double dbl, char *string)
     int slen = 0;
     uint64_t frac;
 
-    frac = (dbl > 0 ? dbl : -dbl) * 100.0;
+    frac = (dbl > 0 ? dbl : -dbl) * 100.0 + 0.5;
     frac %= 100;
 
     /* write decimal part to string */