set public header include dir for fixing header prefix

[jhlee525]

In the previous PR(#297), the install directory for cbor.h was changed to tinycbor/cbor.h.

it causes problem when using FetchContent, since it import headers directly from the source code. (It actually placed in src/tinycbor.h).

I created a dedicated public header directory include by convention, and placed include/tinycbor/cbor.h.

Since all source files reference just cbor.h, I have to updated all include paths accordingly.

I checked tests and tools are able to compile, but not tested for all option sets.

Apologies for the big changes, If you prefer not to adopt this change, feel free to revert #297.