[gles/cogl-shader] Fix parameter spec for cogl_shader_get_info_log

The stub version of cogl_shader_get_info_log needed to be updated to
match the changes to the function signature in 61deeafa.

gles/cogl-shader.c

@@ -200,11 +200,10 @@ cogl_shader_compile (CoglHandle shader_handle)
 {
 }
 
-void
-cogl_shader_get_info_log (CoglHandle  handle,
-                          size_t      size,
-                          char       *buffer)
+gchar *
+cogl_shader_get_info_log (CoglHandle handle)
 {
+  return NULL;
 }
 
 CoglShaderType