fixes broken PEcAn BUILD 46efcd035e18337f0fbe53d426bbf6c779bce544

error causing break was 

```
----------------------------------------------------------------------
INSTALL db BROKEN
----------------------------------------------------------------------
* installing *source* package 'PEcAn.DB' ...
** R
Error in .install_package_code_files(".", instdir) :
files in '/home/kooper/autobuild/pecan/db/R' missing from 'Collate' field:
  db.exists.R
ERROR: unable to collate and parse R files for package 'PEcAn.DB'
* removing '/home/kooper/lib/R/PEcAn.DB'
```