gdb/copyright.py: Add update-netbsd.sh to MULTIPLE_COPYRIGHT_HEADERS

Add gdb/syscalls/update-netbsd.sh to the reminder printed
at the end of the execution listing all the files where
a manual update of the copyright header is needed. This
scripts contains some inline code which includes a copyright
header.
diff --git a/gdb/copyright.py b/gdb/copyright.py
index 560754f..c4e6f94 100644
--- a/gdb/copyright.py
+++ b/gdb/copyright.py
@@ -245,6 +245,7 @@
     "gdb/doc/gdb.texinfo",
     "gdb/doc/refcard.tex",
     "gdb/gdbarch.sh",
+    "gdb/syscalls/update-netbsd.sh",
 )
 
 # The list of file which have a copyright, but not head by the FSF.