frameworks/native/opengl: Fix build with gcc 4.8

In gcc 4.8, the warning for unused parameters (such as JNIEnv* pointers
in JNI functions that don't need it) is enabled by default - causing
-Werror to break the build.

Change-Id: Ib91721f2cb9912b79c2ac19b4210b1309b08304f
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>