cogl: Revert "build: Remove unused variable"

Having ctx here produces a warning on GLES. However it's needed for Big
GL as we have at the top of the file:

 #ifdef HAVE_COGL_GL
 #define glClientActiveTexture ctx->drv.pf_glClientActiveTexture
 #endif

This reverts commit 27a3a2056a.

cogl/cogl.c

@@ -705,6 +705,8 @@ cogl_read_pixels (int x,
 static void
 _cogl_disable_other_texcoord_arrays_cb (int texcoord_array_num, gpointer data)
 {
+  CoglContext *ctx = data;
+
   GE (glClientActiveTexture (GL_TEXTURE0 + texcoord_array_num));
   GE (glDisableClientState (GL_TEXTURE_COORD_ARRAY));
 }