Remove consts from defined structs to avoid errors with c++

[sebirdman]

This should properly resolve #51

[AppVeyorBot]

❌ Build fff 85-appveyor failed (commit b27f8d19ad by @sebirdman)

[AppVeyorBot]

❌ Build fff 86-appveyor failed (commit 6fb172869f by @sebirdman)

[sebirdman]

I'm not sure what the tests are doing exactly, i'm not able to reproduce these failures on my machine

[johwech]

The changes introduced by the original pull request require the fff.h to be not included within an extern "C" block when compiling with a cpp compiler.

I fixed the issue by wrapping an additional extern "C++" block around the whole fff.h:
https://ci.appveyor.com/project/johwech/fff/builds/32597466

Alternatively it's also possible to just fix the test and tell the users that fff.h cannot be included within an extern "C" segment when a CPP compiler is used:
https://ci.appveyor.com/project/johwech/fff/builds/32597324

[gndl]

Hi,
first of all thank you for this great tool.
This PR allows me to use FFF, how can I help to get it integrated mainstream?