CMake: dxgsg9's USE_GENERIC_DXERR_LIBRARY should be public

The reason is that it's used in the public headers, so we
need to make sure that users of the public headers also
use the same preprocessor definition.

panda/src/dxgsg9/CMakeLists.txt

@@ -40,7 +40,7 @@ add_component_library(p3dxgsg9 SYMBOL BUILDING_PANDADX
   ${P3DXGSG9_HEADERS} ${P3DXGSG9_SOURCES})
 target_link_libraries(p3dxgsg9 p3windisplay panda
   PKG::CG PKG::DX9)
-target_compile_definitions(p3dxgsg9 PRIVATE USE_GENERIC_DXERR_LIBRARY) # FIXME
+target_compile_definitions(p3dxgsg9 PUBLIC USE_GENERIC_DXERR_LIBRARY) # FIXME
 
 if(NOT BUILD_METALIBS)
   install(TARGETS p3dxgsg9 DESTINATION lib RUNTIME DESTINATION bin)