[nmi_gen] Ignore intg_err_o output from reg_top

This signal got added by commit 915df69, but we didn't wire it up
properly here (causing a Verilator lint error).

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/hw/ip/nmi_gen/rtl/nmi_gen.sv b/hw/ip/nmi_gen/rtl/nmi_gen.sv
index ec16a08..d8e68db 100644
--- a/hw/ip/nmi_gen/rtl/nmi_gen.sv
+++ b/hw/ip/nmi_gen/rtl/nmi_gen.sv
@@ -43,6 +43,7 @@
     .tl_o,
     .reg2hw,
     .hw2reg,
+    .intg_err_o(),
     .devmode_i(1'b1)
   );