[build] Fixes a cogl-vertex-buffer.c warning when building for GLES
[build] Fixes a cogl-vertex-buffer.c warning when building for GLES

GLES 1 doesn't support GLSL so it never needs to use the generic_index
variable for generic attributes which was flagging a warning.

djdeath/cogl,Distrotech/cogl,spatulasnout/cogl,gcampax/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl-android,gcampax/cogl,spatulasnout/cogl,Distrotech/cogl,collects/cogl,Distrotech/cogl,Distrotech/cogl,djdeath/cogl-android,djdeath/cogl-android,collects/cogl,djdeath/cogl,djdeath/cogl