Ensure files referencing ssize_t include sys/types.h

To define ssize_t on POSIX systems, sys/types.h should be included and Windows requires typedefs in bson-compat.h. Ensure that files in libmongoc that are referencing ssize_t are including both. Currently it appears that files in libmongoc get sys/types.h through bson.h => bson-types.h

There are plenty of libmongoc source files that utilize ssize_t without directly including this <sys/types.h>. This appears to work because they all end up including
bson.h, which in turn includes bson-types.h, which happens to include <sys/types.h>. But if a libmongoc source file was introduced that happened to use ssize_t and not depend on bson.h, it may lead to the a compilation error.