update-mocks: don't try to regenerate files.

If you do update-mocks in a dirty tree, the recursive make that it
uses will try to rebuild things!  Suppress that.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tools/update-mocks.sh

@@ -13,7 +13,7 @@ START=$(grep -F -n '/* AUTOGENERATED MOCKS START */' "$FILE" | cut -d: -f1)
 END=$(grep -F -n '/* AUTOGENERATED MOCKS END */' "$FILE" | cut -d: -f1)
 
 function make_binary() {
-    $MAKE "${FILE/%.c/}" 2> "${BASE}.err" >/dev/null
+    $MAKE SUPPRESS_GENERATION=1 "${FILE/%.c/}" 2> "${BASE}.err" >/dev/null
 }
 
 if [ -n "$START" ]; then