build/xilinx/vivado: only set library for vhdl files (not supported for verilog/system-verilog)

litex/build/xilinx/vivado.py

@@ -1,4 +1,4 @@
-# This file is Copyright (c) 2014 Florent Kermarrec <florent@enjoy-digital.fr>
+# This file is Copyright (c) 2014-2019 Florent Kermarrec <florent@enjoy-digital.fr>
 # License: BSD
 
 import os
@@ -118,6 +118,7 @@ class XilinxVivadoToolchain:
             for filename, language, library in sources:
                 filename_tcl = "{" + filename + "}"
                 tcl.append("add_files " + filename_tcl)
+                if language == "vhdl":
                     tcl.append("set_property library {} [get_files {}]"
                                .format(library, filename_tcl))
         for filename in edifs: