mdadm: Do not check RUN_DIR

mdadm: Do not check RUN_DIR

Fixes build failure on hosts that do not have mdadm
installed/configured:

make[3]: Entering directory
`/local/users/fainelli/openwrt/trunk/build_dir/target-mipsel-linux-gnu_glibc/mdadm-4.0'
***** Parent of /run/mdadm does not exist.  Maybe set different RUN_DIR=
*****  e.g. make RUN_DIR=/dev/.mdadm
***** or set CHECK_RUN_DIR=0
make[3]: *** [check_rundir] Error 1
make[3]: Leaving directory
`/local/users/fainelli/openwrt/trunk/build_dir/target-mipsel-linux-gnu_glibc/mdadm-4.0'
make[2]: ***
[/local/users/fainelli/openwrt/trunk/build_dir/target-mipsel-linux-gnu_glibc/mdadm-4.0/.built]
Error 2
make[2]: Leaving directory
`/local/users/fainelli/openwrt/trunk/package/utils/mdadm'
make[1]: *** [package/utils/mdadm/compile] Error 2
make[1]: Leaving directory `/local/users/fainelli/openwrt/trunk'
make: *** [package/mdadm/compile] Error 2

Fixes: 980c41f8e04f ("utils/mdadm: Update to 4.0")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>