Add Unity generated meta file for an already ignored debugging file

[jrreed]

Reasons for making this change:

Unity generates and manages it's own *.*.meta metatdata files for every file and directory in a Unity project. These files must be added/removed/renamed along with their respective file/directory.

*.pdb files (generated by Visual Studio for debugging in Windows) were ignored in #2240, but this did not include their corresponding *.pdb.meta files.

Similarly, *.pidb and their corresponding *.pidb.meta files were ignored in #1319.

Without this, Unity will complain that *.pdb.meta files exist when a corresponding *.pdb does not exist and remove the offending *.pdb.meta files anyways. Committing these removals will cause them to be re-generated on the developers machine who has the *.pdb files. This causes unnecessary add/remove fighting in the codebase.

Links to documentation supporting these rule changes:

Visual Studio *.pdb debugging files
Unity *.*.meta metadata files
Unity *.*.meta metadata files with respect to Version Control

[shiftkey]

@jrreed thanks!