fix rc ptr constructor emplace warnings

Author: 9il

source/mir/rc/ptr.d

@@ -137,7 +137,7 @@ struct mir_rcptr(T)
             } ();
             import mir.functional: forward;
             import mir.conv: emplace;
-            emplace!T(_value, forward!args);
+            cast(void) emplace!T(_value, forward!args);
         }
     }
 }

source/mir/rc/slim_ptr.d

@@ -133,7 +133,7 @@ struct mir_slim_rcptr(T)
             } ();
             import mir.functional: forward;
             import mir.conv: emplace;
-            emplace!T(_value, forward!args);
+            cast(void) emplace!T(_value, forward!args);
         }
     }
 }