fix prototype for 8.1

[remicollet]

Without:

TEST 51/51 [tests/unbuffered001.phpt]
========DIFF========
001+ Deprecated: Declaration of pq\DateTime::createFromFormat($format, $datetime, ?DateTimeZone $object = <default>) should be compatible with DateTime::createFromFormat(string $format, string $datetime, ?DateTimeZone $timezone = null): DateTime|false in Unknown on line 0
002+ 
003+ Deprecated: Declaration of pq\DateTime::jsonSerialize() should be compatible with JsonSerializable::jsonSerialize(): mixed in Unknown on line 0
     Test
     bool(false)
     bool(true)
--
========DONE========
FAIL unbuffered result [tests/unbuffered001.phpt] 

...

Tests failed    :   51 (100.0%) (100.0%)
Tests passed    :    0 (  0.0%) (  0.0%)

With PHP 8.1.0alpha1 and this patch

=====================================================================
TEST RESULT SUMMARY
---------------------------------------------------------------------
Exts skipped    :    0
Exts tested     :   64
---------------------------------------------------------------------

Number of tests :   51                51
Tests skipped   :    0 (  0.0%) --------
Tests warned    :    0 (  0.0%) (  0.0%)
Tests failed    :    0 (  0.0%) (  0.0%)
Tests passed    :   51 (100.0%) (100.0%)
---------------------------------------------------------------------
Time taken      :    1 seconds
=====================================================================

[codecov]

Codecov Report

Merging #40 (aaf7e89) into master (7109996) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #40   +/-   ##
=======================================
  Coverage   76.61%   76.61%           
=======================================
  Files          17       17           
  Lines        3592     3592           
=======================================
  Hits         2752     2752           
  Misses        840      840           

Impacted Files	Coverage Δ
src/php_pq_misc.c	72.63% <ø> (ø)

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