Add missing clInitLayerWithProperties return type

In PR KhronosGroup#1350 which added
the `clInitLayerWithProperties` entry-point to cl_loader_layers the
`cl_int` return type was missing from the entry-point declaration
snippets.

Author: EwanC

extensions/cl_loader_layers.asciidoc

@@ -59,6 +59,7 @@ strategies for backward and forward compatibility.
 
 [source,opencl]
 ----
+cl_int
 clInitLayerWithProperties(cl_uint                    num_entries,
                           const cl_icd_dispatch     *target_dispatch,
                           cl_uint                   *num_entries_ret,
@@ -216,6 +217,7 @@ If they require deinitialization, layers that support
 
 [source,opencl]
 ----
+cl_int
 clInitLayerWithProperties(cl_uint                    num_entries,
                           const cl_icd_dispatch     *target_dispatch,
                           cl_uint                   *num_entries_ret,