Only set the public headers path for the iOS static lib.

Instead of applying the public headers path to all build configurations, only apply it to the targets where it is needed. This should retain the behaviour intended in https://github.com/libgit2/objective-git/commit/36ec513f7188a02b3d098da470d4d3d2d4c437fe (putting the headers in the built products dir instead of /usr/local) while correcting the problem where headers weren’t being placed into the Mac framework bundle, as described here: https://github.com/libgit2/objective-git/issues/344#issuecomment-38632852