ADDON_FRAMEWORKS on iOS broken

[danomatika]

Moving issue from openframeworks/openFrameworks#2412

Generating an iOS project with frameworks results in a corrupted project which gives the following error:

Project /Users/dano/src/openFrameworks/addons/ofxMidi/exampleIOS/exampleIOS.xcodeproj 
cannot be opened: -[PBXBuildFile group]: unrecognized selector sent to instance
0x7f9202bf22a0

This is from testing the exampleIOS project in ofxMidi.

Also, digging into the iOS framework issue, I noticed that the wrong path is being set:

/System/Library/Frameworks/CoreMIDI.framework

This works fine on OS X, but the path is wrong for iOS since it looks for relative paths inside the iOS SDK folder. It should be the same without the beginning slash:

System/Library/Frameworks/CoreMIDI.framework

This is set on line 1042 in xcodeProject.cpp.