mime.bbclass: fix post install scriptlet error

fix error during post uninstall:
%postun(shared-mime-info-data-2.0-r0.4.corei7_64): execv(/bin/sh) pid 78
+ '[' 0 = 0 ']'
+ set -e
+ '[' x '!=' x ']'
+ echo 'Updating MIME database... this may take a while.'
Updating MIME database... this may take a while.
+ update-mime-database /usr/share/mime
Directory '/usr/share/mime/packages' does not exist!
%postun(shared-mime-info-data-2.0-r0.4.corei7_64): waitpid(78) rc 78 status 100
warning: %postun(shared-mime-info-data-2.0-r0.4.corei7_64) scriptlet failed, exit status 1

when run post uninstall scriptlet, /usr/share/mime/packages has been
removed during unintall, while update-mime-database will check xml under
/usr/share/mime/packages.

workaround by create this dir before update, then remove it

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 6f262a31)
Signed-off-by: Steve Sakoman <steve@sakoman.com>