Validate that a specification's `public_header_files` and `private_header_files` file patterns only match header files.
Also, validate that all file patterns, if given, match at least one file.
Merge pull request #2917 from CocoaPods/seg-validate-file-patterns

Validate that a specification's `public_header_files` and `private_header_files` file patterns only match header files.
[Validator] Use header extensions constant from FileAccessor