build: Generate the export header
* src/CMakeLists.txt: Remove compile definition BUILDING_QGPGME. Remove qgpgme_export.h from qgpgme_PRIVATE_HEADERS. Use generate_export_header to generate qgpgme_export.h. Install the generated header. * src/qgpgme_export.h: Remove.
This ensures that we have correct EXPORT and DEPRECATED macros for all
platforms.
- GnuPG-bug-id: T7262