Fix import path argument

If an `imports_dir` argument is specified to `protobuffs_compile:generate_source/2`,
the pathname was being added as individual characters.  (The string is being appended to the list as a list of characters).  Wrap the argument in brackets to insert as a string.


```erlang
protobuffs_compile:generate_source("myfile.proto", [{imports_dir, "proto/"}]).
% gave
ImportPath == [ "./", "src/", 112, 114, 111, 116, 111, 47 ]
% after this change: 
ImportPath == [ "./", "src/", "proto/" ]
```