Add generate_export_header to be able compile with -DBUILD_SHARED_LIBS=ON

[jmlich]

It seems that generate_export_header(QtAES EXPORT_MACRO_NAME QTAESSHARED_EXPORT) was removed in commit 93020ab.

This causes issues when compiling the project with the -DBUILD_SHARED_LIBS=ON CMake flag. We use Qt-AES in Amazfish piggz/harbour-amazfish#520, and we are trying to package it for Alpine Linux, which requires -DBUILD_SHARED_LIBS=ON to be enabled for such packages.

I wasn’t able to find an explanation for why it was removed.

It seems to have been either only partially removed or removed by mistake.

The generate_export_header macro generates the qtaes_export.h file, which is included here:

Qt-AES/qaesencryption.h

Line 5 in e55c81b

#include "qtaes_export.h"

I have added it back to CMake. The other option would be to remove the #include in qaesencryption.h.

Fixes: #68