sunxi-mali: add missing GLchar definitions.

The GLES and GLES2 header files provided by sunxi-mali do not define the
type "GLchar" and use "char" instead in the prototype of some functions.

In order to have some applications build (like cairo), a new patch has
been added to define the missing type.

This issue has been reported upstream some time ago, but the pull
request has not been merged yet.
(See https://github.com/linux-sunxi/sunxi-mali/pull/8).

Fixes http://autobuild.buildroot.net/results/2cb/2cb13a5bb92dabed219d49f49f0b9a2dfe65a0ca/

.

[Thomas: add more details to the patch description, using the commit
log message details.]

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>