Wholesale conversion to #pragma once

Kudos to cgmb and https://github.com/cgmb/guardonce

Change-Id: Ifa8970734b8d43fd08c9260c645bdb0228633791
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/libs/cplusplus/PreprocessorEnvironment.h

@@ -43,8 +43,7 @@
   CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */
 
-#ifndef PREPROCESSORENVIRONMENT_H
-#define PREPROCESSORENVIRONMENT_H
+#pragma once
 
 #include "PPToken.h"
 
@@ -101,5 +100,3 @@ private:
 };
 
 } // namespace CPlusPlus
-
-#endif // PREPROCESSORENVIRONMENT_H