Fix a plugin loader runtime failure (#2061)

The plugins must export the symbols to allow loading by the panel
executable.
CMAKE_EXECUTABLE_ENABLE_EXPORTS is too new for us.

Supersedes #2053.
Fixes #2052.

panel/CMakeLists.txt

@@ -107,6 +107,8 @@ target_link_libraries(${PROJECT}
     ${STATIC_PLUGINS}
 )
 
+set_property(TARGET ${PROJECT} PROPERTY ENABLE_EXPORTS TRUE)
+
 install(TARGETS ${PROJECT} RUNTIME DESTINATION bin)
 install(FILES ${CONFIG_FILES} DESTINATION ${CMAKE_INSTALL_DATADIR}/lxqt)
 install(FILES ${PUB_HEADERS} DESTINATION include/lxqt)