meson: Fix argument for makensis (build regression)

`make installer` with a DLL directory was broken.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Message-Id: <20201117190640.390359-1-sw@weilnetz.de>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/scripts/nsis.py b/scripts/nsis.py
index e1c4093..5135a05 100644
--- a/scripts/nsis.py
+++ b/scripts/nsis.py
@@ -65,7 +65,7 @@
             dlldir = "w64"
             makensis += ["-DW64"]
         if os.path.exists(os.path.join(args.srcdir, "dll")):
-            makensis += "-DDLLDIR={0}/dll/{1}".format(args.srcdir, dlldir)
+            makensis += ["-DDLLDIR={0}/dll/{1}".format(args.srcdir, dlldir)]
 
         makensis += ["-DOUTFILE=" + args.outfile] + args.nsisargs
         subprocess.run(makensis)