CDRIVER-4789 libbson: prevent -Werror=conversion with GCC 12 (#1479)

* libbson: prevent -Werror=conversion with GCC 12

Building fails with GCC 12.3:

    bson-iter.h:434:33: error: conversion from 'int64_t' {aka 'long long int'} to '__suseconds_t' {aka 'long int'} may change value [-Werror=conversion]
    434 | tv->tv_usec = (value % 1000) * 1000;
    | ~~~~~~~~~~~~~~~^~~~~~
    cc1plus: all warnings being treated as errors

Do the same as with tv->tv_sec, and explicitely cast it to suseconds_t
on non-Win32 systems and to long on Win32.

* use `time_t` in assignment to `tv_sec`

This matches specificiation in POSIX 2008.

src/libbson/NEWS

@@ -1,7 +1,9 @@
 libbson 1.25.2 (Unreleased)
 ===========================
 
-TODO: Add news.
+Fixes:
+
+ * Fix conversion warning with GCC 12.
 
 libbson 1.25.1
 ==============

src/libbson/src/bson/bson-iter.h

@@ -440,10 +440,11 @@ bson_iter_timeval_unsafe (const bson_iter_t *iter, struct timeval *tv)
  int64_t value = bson_iter_int64_unsafe (iter);
 #ifdef BSON_OS_WIN32
  tv->tv_sec = (long) (value / 1000);
+ tv->tv_usec = (long) (value % 1000) * 1000;
 #else
- tv->tv_sec = (suseconds_t) (value / 1000);
+ tv->tv_sec = (time_t) (value / 1000);
+ tv->tv_usec = (suseconds_t) (value % 1000) * 1000;
 #endif
- tv->tv_usec = (value % 1000) * 1000;
 }