QGCApplication: fix missing QT_QUICK_CONTROLS_STYLE in linux systems

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

src/QGCApplication.cc

@@ -28,6 +28,7 @@
 #include <QFontDatabase>
 #include <QQuickWindow>
 #include <QQuickImageProvider>
+#include <QQuickStyle>
 
 #ifdef QGC_ENABLE_BLUETOOTH
 #include <QBluetoothLocalDevice>
@@ -218,6 +219,11 @@ QGCApplication::QGCApplication(int &argc, char* argv[], bool unitTesting)
             }
             permFile.close();
         }
+
+        // Set default QtQuick style if not configured
+        if (QString(getenv("QT_QUICK_CONTROLS_STYLE")).isEmpty()) {
+            QQuickStyle::setStyle("Universal");
+        }
     }
 #endif
 #endif