package/ncurses: slight code re-arrangement

Move the definition of libraries to install before it is actually used.

Also, in a coming changeset, it will also be used to know which
pkg-config files to symlink.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>