TypeCategory: Initialize m_enabled to false

The initialization was accidentally lost in https://reviews.llvm.org/D71310, causing a ubsan failure:

/Users/buildslave/jenkins/workspace/lldb-cmake-sanitized/llvm-project/lldb/include/lldb/DataFormatters/TypeCategory.h:278:35: runtime error: load of value 190, which is not a valid value for type 'bool'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /Users/buildslave/jenkins/workspace/lldb-cmake-sanitized/llvm-project/lldb/include/lldb/DataFormatters/TypeCategory.h:278:35 in

http://lab.llvm.org:8080/green/view/LLDB/job/lldb-cmake-sanitized/621/consoleText