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.  Wrap the argument in an
array to insert as a string.

protobuffs_compile:generate_source("myfile.proto", [{imports_dir, "proto/"}]).
yielded ImportPath = [ "./", "src/", 112, 114, 111, 116, 111, 47 ]
After the fix, ImportPath = [ "./", "src/", "proto/" ]