package/redis: fix static linking with libatomic

Fixes
http://autobuild.buildroot.net/results/7f1/7f1ecccbfdb6bd95824d9c884f1577e71e0e1e09/
http://autobuild.buildroot.net/results/c0b/c0b1bdcc5fbddf8b996b923015184d753882d4b8/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas:
 - improve comment to explain the fix
 - remove useless LDFLAGS related code]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>