Makefile: fix "version.h" build for cross-compiling

Makefile: fix "version.h" build for cross-compiling

When using "make CROSS=mipsel-linux-", I get the following errors:

/bin/sh: /home/norris/git/mtd-utils/mipsel-linux/include/version.h.tmp: No such file or directory
make: *** [/home/norris/git/mtd-utils/mipsel-linux/include/version.h.tmp] Error 1

Fix (suggested by Mike Frysinger): create the directory if it doesn't
exist. Also, $(CC) needs to be able to find the generated header in
$(BUILDDIR)/include.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>