Add SPDX license headers

apps/lib/diff/diffeditor.cpp

+/*
+    SPDX-FileCopyrightText: 2022 Waqar Ahmed <waqar.17a@gmail.com>
+    SPDX-License-Identifier: MIT
+*/
 #include "diffeditor.h"
 #include "difflinenumarea.h"
 #include "diffwidget.h"

apps/lib/diff/diffeditor.h

+/*
+    SPDX-FileCopyrightText: 2022 Waqar Ahmed <waqar.17a@gmail.com>
+    SPDX-License-Identifier: MIT
+*/
 #ifndef KATE_DIFF_EDITOR
 #define KATE_DIFF_EDITOR
 

apps/lib/diff/difflinenumarea.cpp

+/*
+    SPDX-FileCopyrightText: 2022 Waqar Ahmed <waqar.17a@gmail.com>
+    SPDX-License-Identifier: MIT
+*/
 #include "difflinenumarea.h"
 #include "diffeditor.h"
 

apps/lib/diff/difflinenumarea.h

+/*
+    SPDX-FileCopyrightText: 2022 Waqar Ahmed <waqar.17a@gmail.com>
+    SPDX-License-Identifier: MIT
+*/
 #ifndef DIFF_LINE_NUM_AREA
 #define DIFF_LINE_NUM_AREA
 

apps/lib/diff/diffparams.h

+/*
+    SPDX-FileCopyrightText: 2022 Waqar Ahmed <waqar.17a@gmail.com>
+    SPDX-License-Identifier: MIT
+*/
 #ifndef KATE_DIFF_PARAMS
 #define KATE_DIFF_PARAMS