#123 import OpenGL.GL.SGIS.generate_mipmap broken

[Nerull]

The alias definition appears to be backwards, causing
an exception. When changed to:

glInitGenMipmapSGIS = glInitGenerateMipmapSGIS
from
glInitGenerateMipmapSGIS = glInitGenMipmapSGIS

extension seems to work fine afterwards.