Fix fallback for bson_get_monotonic_time
Fix fallback for bson_get_monotonic_time

bson_get_monotonic_time incorrectly called bson_gettimeofday with a
second parameter when invoked on a platform without a monotonic clock.

This fixes that call.

Closes #108

alexeyvo/libbson,Convey-Compliance/libbson,Convey-Compliance/libbson,alexeyvo/libbson,alexeyvo/libbson,Convey-Compliance/libbson