This plugin for Sublime Text 2/3 adds completion for include files. That is, if completion is triggered while the cursor is within a C/C++ include statement, it offers completions for available files. This is based on the include paths given by -I command line arguments in some setting (configurable). It is recommended to use this plugin in combination with Clang-Complete or SublimeClang, though not required.
Just clone this git repo into your Packages folder.