core: fix arithmetic on a pointer to void

Use char* instead of void* for this
pointer arithmetic. Fixes the following:

error: arithmetic on a pointer to void
buf = buf + offset;
      ~~~ ^

Change-Id: I6cfc92c9240fda66a52e780cda7820fdba76914a

core/jni/com_google_android_gles_jni_GLImpl.cpp

@@ -183,7 +183,7 @@ getDirectBufferPointer(JNIEnv *_env, jobject buffer) {
             if (array) {
                 releasePointer(_env, array, buf, 0);
             }
-            buf = buf + offset;
+            buf = (char*)buf + offset;
         } else {
             jniThrowException(_env, "java/lang/IllegalArgumentException",
                               "Must use a native order direct Buffer");