add missed MANIFEST entries

Author: mohawk2

MANIFEST

@@ -1,3 +1,4 @@
+Changes
 COPYRIGHT
 FreeGLUT/freeglut-Readme.txt
 FreeGLUT/freeglut.dll
@@ -17,6 +18,7 @@ include/GL/gl.h
 include/GL/glext.h
 include/GL/glprocs.h
 include/GL/glu.h
+include/GL/glu_mangle.h
 include/GL/glut.h
 include/GL/glxext.h
 include/GL/wglext.h