[osg] Fix feature plugin

[JackBoosY]

When building osg[plugin]:

installed\x86-windows\include\Inventor/system/gl-headers.h(50,10): fatal error C1083: Cannot open include file: 'GL/glext.h': No such file or directory (compiling source file D:\buildtrees\osg\src\raph-3.6.5-4fd22a77bc.clean\src\osgPlugins\Inventor\ConvertFromInventor.cpp) [D:\buildtrees\osg\x86-windows-dbg\src\osgPlugins\Inventor\osgdb_iv.vcxproj]

Fix this issue.

[JackBoosY]

coin:Inventor/system/gl-headers.h
There is no dependency between coin and osg, but why it happend?

Edit: Seems use wrong opengl headers:

coin:Inventor/system/gl-headers.h
coin:Inventor/system/gl.h
coin:Inventor/system/inttypes.h

Edit:
In osg\src\CMakeLists.txt line 743:

FIND_PACKAGE(Inventor)

This finds the coin, and the coin needs to use the opengl-registry header file.
In osg\src\osgPlugins\CMakeLists.txt line 158:

IF(NOT OSG_GLES1_AVAILABLE AND NOT OSG_GLES2_AVAILABLE)
    IF(INVENTOR_FOUND)
        ADD_PLUGIN_DIRECTORY(Inventor)
    ENDIF()
ENDIF()

This use coin in the plugin Inventor.

[JackBoosY]

Inventor is a part of coin.

[BillyONeal]

Thanks for your contribution!