coqPackages.ssreflect: add ncurses as a dependency

This is needed in the Darwin pure environment, where ncurses is not
available within the sandbox.  The exact error was:

coqmktop -coqlib `coqtop -where` -o bin/ssrcoq.byte -I +threads src/ssrmatching.cmo src/ssreflect.cmo
ld: warning: directory not found for option '-L/nix/store/wlf8pb7gz5alzbhdxwayxx0x4as3rnw8-coq-8.5b2/lib/coq/kernel/byterun'
ld: library not found for -lncurses