kbuild: replace hardcoded bison in cmd_bison_h with $(YACC)

Commit 73a4f6db ("kbuild: add LEX and YACC variables") missed to
update cmd_bison_h somehow.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.lib

@@ -196,7 +196,7 @@ $(obj)/%.tab.c: $(src)/%.y FORCE
 	$(call if_changed,bison)
 
 quiet_cmd_bison_h = YACC    $@
-      cmd_bison_h = bison -o/dev/null --defines=$@ -t -l $<
+      cmd_bison_h = $(YACC) -o/dev/null --defines=$@ -t -l $<
 
 $(obj)/%.tab.h: $(src)/%.y FORCE
 	$(call if_changed,bison_h)