cogl-pipeline-glsl: Use char* instead of GLchar*

It appears that some GLES2 headers don't define GLchar and it works
just as well to use a regular char.

cogl/cogl-pipeline-glsl.c

@@ -1039,7 +1039,7 @@ _cogl_pipeline_backend_glsl_end (CoglPipeline *pipeline,
 
       if (glsl_program_state->source)
         {
-          const GLchar *source_strings[2];
+          const char *source_strings[2];
           GLint lengths[2];
           GLint compile_status;
           GLuint shader;