Silence switch-default error in docopt_value.h

When a project using docopt.cpp is built with `-Werror=switch-default`
option, there's an error
```
error: switch missing default case [-Werror=switch-default]
```
which this fixes.

Signed-off-by: Eero Aaltonen <eero.aaltonen@vaisala.com>

Author: Eero Aaltonen

docopt_value.h

@@ -77,7 +77,9 @@ namespace docopt {
 			std::string strValue;
 			std::vector<std::string> strList;
 		};
-		
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wswitch-default"
 		static const char* kindAsString(Kind kind) {
 			switch (kind) {
 				case Kind::Empty: return "empty";
@@ -88,6 +90,7 @@ namespace docopt {
 			}
 			return "unknown";
 		}
+#pragma GCC diagnostic pop
 
 		void throwIfNotKind(Kind expected) const {
 			if (kind_ == expected)