redis: needs dynamic library

dlfcn.h is included in src/debug.c and src/module.c
module.c uses dlopen function

Fixes:
 - http://autobuild.buildroot.org/results/fdda0ee7461dfc9ea479d4d80ab42c50006a3b13



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>