docs: add necessary headers to Documentation/MFOW.txt

docs: add necessary headers to Documentation/MFOW.txt

The tutorial in Documentation/MyFirstObjectWalk.txt
contains the functions trace_printf(), oid_to_hex(),
and pp_commit_easy(), and struct oidset, which are used
without any hint of where they are defined. When the provided
code is compiled, the compiler returns an error, stating that
the functions and the struct are used before declaration. Therefore,include
necessary header files (the ones which have no mentions in the tutorial).

Signed-off-by: Vinayak Dev <vinayakdev.sci@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>