Environment: Mark hasKey(...) method as const

Change-Id: I8f355e6ead9136741af199d08935a0f3e7446110
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/libs/utils/environment.cpp

@@ -391,7 +391,7 @@ QList<EnvironmentItem> Environment::diff(const Environment &other) const
     return result;
 }
 
-bool Environment::hasKey(const QString &key)
+bool Environment::hasKey(const QString &key) const
 {
     return m_values.contains(key);
 }