Debugger: Always add Qt install source mappings for gdb.

As it fails for MinGW.

Task-number: QTCREATORBUG-4132
Reviewed-by: hjk

src/plugins/debugger/debuggerstartparameters.h

@@ -96,6 +96,7 @@ public:
     QString projectBuildDir;
     QString projectDir;
 
+    QString qtInstallPath;
     // Used by remote debugging.
     QString remoteChannel;
     QString remoteArchitecture;