fix compile breakage with "make -j"

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/Makefile b/Makefile
index 9a82776..ef7c532 100644
--- a/Makefile
+++ b/Makefile
@@ -512,6 +512,8 @@
 include/autoconf.h: .kconfig.d .config $(wildcard $(srctree)/*/*.c) $(wildcard $(srctree)/*/*/*.c) | gen_build_files
 	$(Q)$(MAKE) -f $(srctree)/Makefile silentoldconfig
 
+include/usage.h: gen_build_files
+
 else
 # Dummy target needed, because used as prerequisite
 include/autoconf.h: ;