Don't duplicate SPDX headers

add_license.sh

 #!/bin/bash
 
 find "$@" -name '*.h' -o -name '*.cpp' -o -name '*.qml' | grep -v /3rdparty/ | while read FILE; do
+    if grep -qiE "SPDX" "$FILE"; then continue; fi
     if grep -qiE "Copyright \(C\) [0-9, -]{4,} " "$FILE" ; then continue; fi
     thisfile=`basename $FILE`
     authorName=`git config user.name`