pkg-kconfig: add missing -patch dependency for defconfig

Since the introduction of _KCONFIG_DEFCONFIG in 8ef62b99, the package's
.config file no longer depends on anything (unless a fragment is
defined). Therefore, there is no dependency anymore between .config
and <pkg>-patch. Thus, it is possible that the .config file is
attempted to be built before the package is extracted and patched.
Usually this works out OK because <pkg>-patch will always be done
before <pkg>-configure, but it will fail when the user calls
<pkg>-menuconfig explicitly. It will also fail when we enable
top-level parallel build.

To solve this, just add an explicit order-only dependency on
<pkg>-patch. It really is only necessary when _KCONFIG_DEFCONFIG is
defined and _KCONFIG_FRAGMENT_FILES is not, but it doesn't hurt to
add it unconditionally.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reported-by: FrAnKenStEiN MC <chfakht@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>