Import constness fix from gl3.h into gl2.h.

This change has been made in Khronos SVN for GLES2 already, however it
is mildly controversial and discussion is on-going.

This change is necessary for Android because otherwise applications
cannot include both GLES2/gl2.h and GLES3/gl3.h as the glShaderSource()
prototype mismatches. Native programs may include framework headers
(which now include GLES3 headers only) but already have included GLES2
headers, creating an incompatibility.

Change-Id: I70a4075e079ee063e76cef5a227abb0ae70aebb4