devel/libslang2: Add missing LDFLAGS

Without linking libslang2.so.2.3.2 against libncurses a dependant binary
is going to file with Undefined symbol "tgetent" error.

PR: 229126
Submitted by: olevole@olevole.ru
MFH: 2018Q3
Sponsored by: Rubicon Communications, LLC (Netgate)