Fix non-portable use of round().

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 51bc271790eb234a1ba4d14d3e6530f70de92ab5.

Per buildfarm member thrips.

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