[pvr] Fix missing recordings

This fixes an issue with missing recordings that occurs under following
circumstances:

- Recording A in subdirectory "TV Show XY"
- Recording B in subdirectory "TV show xy"

Because of the difference in case, recording B won't show up in any
sub folder. This can happen due to EPG inconsistencies.

The sub folders are created case insensitive (CFileItemList::Contains)
in GetSubDirectories. But when determining if a recording belongs into
a directory, the comparison is case sensitive (in IsDirectoryMember).

This commit changes the IsDirecroryMember check to be case insensitive.