Fix PFNGLBLENDEQUATIONPROC' does not name a type error

Bug described in [1], cherry-picked the commit in [2].

[1] https://github.com/tpaviot/oce/issues/449
[2] https://github.com/tpaviot/oce/commit/da8928f
Merge pull request #1 from chrismullins/cgm_support

Fix PFNGLBLENDEQUATIONPROC does not name a type error