Fix compatibility problems with newer GLES Khronos headers.

Two sets of problems:

- GLchar was missing from "gl.h".
- GLbyte, GLshort, and GLclampf was in both "gl.h" and "glext.h".

This was fine for building Android, but not for other components that
use more of the Khronos functionality.

Test: Compile Android and other code that needs GLchar, GLbyte, GLshort, and GLclampf.
Change-Id: I47ce0c2a5e985a1088cd8e75a94b3573e4aa7f8e