kbuild: fix silentoldconfig with make O=

Al Viro reported that sometimes silentoldconfig failed because
output directory was missing.
So create it unconditionally before executing conf

Signed-off-by: Sam Ravnborg <[e-mail]>

Makefile

···
 # If .config is newer than include/linux/autoconf.h, someone tinkered
 # with it and forgot to run make oldconfig
 include/linux/autoconf.h: .config
+	$(Q)mkdir -p include/linux
 	$(Q)$(MAKE) -f $(srctree)/Makefile silentoldconfig
 else
 # Dummy target needed, because used as prerequisite