cogl: Revert "build: Remove unused variable"
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 27a3a2056a245aca64b8e3a7d82b7976c13e897a.

GNOME/clutter,spatulasnout/clutter,jigpu/clutter,jigpu/clutter,kerrickstaley/clutter-vala,jigpu/clutter,Distrotech/clutter,jigpu/clutter,heysion/clutter-clone,ebassi/clutter,GNOME/clutter,djdeath/clutter,collects/clutter,heysion/clutter-clone,jigpu/clutter,nobled/clutter,spatulasnout/clutter,djdeath/clutter,Distrotech/clutter,jigpu/clutter,nobled/clutter,GNOME/clutter,djdeath/clutter,djdeath/clutter-multithreaded,spatulasnout/clutter,dlespiau/clutter,heysion/clutter-clone,djdeath/clutter-android,GNOME/clutter,ebassi/clutter,djdeath/clutter,collects/clutter,djdeath/clutter,collects/clutter,djdeath/clutter-multithreaded,ebassi/clutter,heysion/clutter-clone,Distrotech/clutter,heysion/clutter-clone,djdeath/clutter-android,djdeath/clutter-multithreaded,dlespiau/clutter,kerrickstaley/clutter-vala,djdeath/clutter-android,Distrotech/clutter,dlespiau/clutter,collects/clutter,spatulasnout/clutter,Distrotech/clutter,djdeath/clutter,djdeath/clutter-android,nobled/clutter,djdeath/clutter-android,spatulasnout/clutter,ebassi/clutter,ebassi/clutter,kerrickstaley/clutter-vala,djdeath/clutter-multithreaded,Distrotech/clutter,GNOME/clutter,collects/clutter,kerrickstaley/clutter-vala