cogl-shader.c: Don't add the common boilerplate twice

cogl-shader.c: Don't add the common boilerplate twice

In 6246c2bd6 I moved the code to add the boilerplate to a shader to a
separate function and also made it so that the common boilerplate is
added as a separate string to glShaderSource. However I didn't notice
that the #define for the vertex and fragment shaders already includes
the common part so it was being added twice. Mesa seems to accept this
but it was causing problems on the IMG driver because COGL_VERSION was
defined twice.