Fix compilation of the project when using Xcode 9 new build system and CocoaPods
Fix compilation of the project when using Xcode 9 new build system and CocoaPods

With current podspec, one .xib file is getting picked up as a source, and is getting into Xcode Compile Sources build step in CocoaPods generated project. 

New build system is more strict than the old one, and prohibits this by saying:

```
error: unexpected duplicate task: CompileXIB /Path-To-Project/Pods/XLPagerTabStrip/Sources/ButtonCell.xib (in target 'XLPagerTabStrip')
```

This behavior is described in [this CocoaPods issue](https://github.com/CocoaPods/CocoaPods/issues/7079).

This PR changes source_files search path from being * to more specific code-related one *.{h,m,swift}, thus picking only sources into sources.

This fix can be verified by installing XLPagerTabStrip, trying to build with new Build System - it will fail. Then you can remove ButtonCell.xib file from Pods -> XLPagerTabStrip -> Compile Sources build phase and this time project will build successfully.
functionaldude/XLPagerTabStrip,xmartlabs/XLPagerTabStrip,xmartlabs/XLPagerTabStrip,functionaldude/XLPagerTabStrip