Fix for MinGW error: 'There are no arguments that depend on a template parameter'

Signed-off-by: Remigiusz Kołłątaj <remigiusz.kollataj@gmail.com>

include/spdlog/sinks/wincolor_sink.h

@@ -44,7 +44,7 @@ public:
 
     virtual ~wincolor_sink()
     {
-        flush();
+        this->flush();
     }
 
     wincolor_sink(const wincolor_sink& other) = delete;