Fix #690 build on Xcode 10 beta

Author: tranngocthachs

src/MacVim/MMPreferenceController.h

@@ -9,7 +9,7 @@
  */
 
 #import <Cocoa/Cocoa.h>
-#import <DBPrefsWindowController.h>
+#import "DBPrefsWindowController.h"
 
 @interface MMPreferenceController : DBPrefsWindowController {
     IBOutlet NSView *generalPreferences;