Fix build_imports

[kohnakagawa]

Hi

This pull request fixes the issue (#517).

The current implementation of build_import_table does not check the correspondence between the imported functions and the trampolines. So, if we add an import function entry by add_entry and build IAT by calling build_import_table, its correspondence is shifted back one by one.

I illustrate this situation.

Before adding a new import entry, the correspondence between IAT and trampolines is correct.

However, after adding wcsrchr (illustrated by orange color), its correspondence is incorrect. In this case, when we try to call puts function, _cexit function is called instead.

To fix this issue, I added a new member manually_added_ to ImportEntry class to distinguish whether the entry is manually added or not. If the entry is a manually-added one, linking between the original IAT entry and the trampoline is skipped.

[romainthomas]

Note: there is this working branch: peimports that tries to improve the way we patch import