package/mono: fix install with parallel builds

Due to a race in the 'install' utility, installing the same file twice
in parallel often results in one of the install calls to fail.

Ensure proper ordering of conflicting rules.

Fixes:
    http://autobuild.buildroot.org/results/c32/c3288c5d1fb94474f14a4a889e76135878d403bc/
    http://autobuild.buildroot.org/results/f70/f708e89f613ec0b42cdf49ccbef39b02f4a271fb/
    http://autobuild.buildroot.org/results/3cb/3cb99e5c5672cbaa2a86020129a05dfde47cdb8f/


    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>