DataFormatters: use include instead of import in C++

'#import' is an Objective-C construct; avoid using it in C++.  NFC.

Addresses PR20867.

Patch by Kevin Avila!

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@217340 91177308-0d34-0410-b5e6-96231b3b80d8