Add pkg-config source file

This allows you to use `pkg_search_module` in CMake to find entityx. Gets around needing a FindEntityX.cmake script. The entity.pc.in file is pretty barebones and could probably be more useful with configurable install prefix dirs, but this catches 99% of installs I would imagine.

Example:

```
include(FindPkgConfig)
pkg_search_module(ENTITYX REQUIRED entityx)

add_executable(some_game main.cpp)
target_link_libraries(some_game ENTITYX_LIBRARIES)
```