ncmpc: fix build with ncursesw

ncmpc: fix build with ncursesw

Fixes:
http://autobuild.buildroot.net/results/c5bb5f978602896179ff9c58ecf4ced40bb96b57

When ncurses is built with wide support, we install libncursesw (plus a
libncurses symlink) but NOT ncursesw.h, only ncurses.h.  This combination
confuses the ncmpc build system as it expects a ncursesw.h when it finds
libncursesw:

meson.build:92:4: ERROR: Problem encountered: No ncursesw header found

The ncurses/ncursesw logic in the build system is only used to know what
header file to #include, and what library to link with.  As we already
provide a libncurses symlink when building with wide support we can simply
force ncmpc to use the ncurses variant.

[Peter: extend commit message]
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>