Fix build failures with clang 12

[alcaeus]

With clang 12, build failures appear due to an implicit fallthrough:

src/bson-encode.c:314:3: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
                case IS_OBJECT: {
                ^
src/bson-encode.c:314:3: note: insert '__attribute__((fallthrough));' to silence this warning
                case IS_OBJECT: {
                ^
                __attribute__((fallthrough)); 
src/bson-encode.c:314:3: note: insert 'break;' to avoid fall-through
                case IS_OBJECT: {
                ^
                break; 
1 error generated.