package/rwmem: needs wchar

rwmmem needs wchar because of fmt dependency since its addition in
commit 51a282bc

Fixes:
 - http://autobuild.buildroot.org/results/1c06c27d70aae8bdb39ae43fbe74896e536293c6



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>