Fix non-portable use of round().

round() is from C99.  Use rint() instead.  There are behavioral
differences between round() and rint(), but they should not matter to
the Bloom filter optimal_k() function.  We already assume POSIX
behavior for rint(), so there is no question of rint() not using
"rounds towards nearest" as its rounding mode.

Cleanup from commit 51bc2717.

Per buildfarm member thrips.

Author: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-Wzn76eCGUonARy-wrVtMHsf+4cvbK_oJAWTLfORTU5ki0w@mail.gmail.com

src/backend/lib/bloomfilter.c

@@ -240,7 +240,7 @@ my_bloom_power(uint64 target_bitset_bits)
 static int
 optimal_k(uint64 bitset_bits, int64 total_elems)
 {
-	int			k = round(log(2.0) * bitset_bits / total_elems);
+	int			k = rint(log(2.0) * bitset_bits / total_elems);
 
 	return Max(1, Min(k, MAX_HASH_FUNCS));
 }