[OpenGL] Ignore Mesa API-mangling headers
[OpenGL] Ignore Mesa API-mangling headers

* Ignore Mesa's {gl,glu,glx}_mangle.h, used for loading both
  a vendor GL and Mesa at the same time.  For now we don't
  support this, and the headers confuse the header parser.


git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@27991 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

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