sim: sh: reduce -Wno-error scope

Only one file in here still generates warnings, so reduce the -Werror
disable to that alone now that we require GNU make and can set variables
on a per-object basis.

sim/sh/Makefile.in

diff --git a/sim/sh/Makefile.in b/sim/sh/Makefile.in
index efac41e..e79efd1 100644
--- a/sim/sh/Makefile.in
+++ b/sim/sh/Makefile.in
@@ -28,8 +28,8 @@
 SIM_EXTRA_CLEAN = sh-clean
 SIM_EXTRA_DEPS = table.c code.c ppi.c
 
-# Code doesn't build cleanly yet.
-SIM_WERROR_CFLAGS =
+# Some modules don't build cleanly yet.
+interp.o: SIM_WERROR_CFLAGS =
 
 ## COMMON_POST_CONFIG_FRAG