mutt: fix build with BR2_PACKAGE_SKELETON_NONE

BR2_PACKAGE_SKELETON_NONE does not provide the /var subdirectory which
breaks symlink creation in that directory. Make sure that /var exists
before creating the symlink.

Fixes:
http://autobuild.buildroot.net/results/32d/32dff0df759baf1120e4f11335e735eb0708b237/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>