ncurses: add SRCHDRS (ncurses_dll.h) to tinfo SRCS

This avoids relying on the includes target to generate this header
which must be in .OBJDIR for the ncurses library to find it.

With an upcoming change libcompat build won't build a seperate
sysroot and will instead share with the 64-bit target which means the
includes target will not be called.

Sponsored by: DARPA, AFRL

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D35510