pinentry: fix ncurses' unsafe header path

pinentry sets the NCURSES_INCLUDE variable in the Makefile to
/usr/include/ncurses and that causes problems when cross-compiling
because it fails with an error like this one:

Making all in curses
make[3]: Entering directory `/br/output/build/pinentry-0.9.0/curses'
/br/output/host/usr/bin/mips64el-ctng_n64-linux-gnu-gcc -DHAVE_CONFIG_H
-I. -I..  -I/usr/include/ncursesw -I../pinentry -D_LARGEFILE_SOURCE
-D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Wall  -D_LARGEFILE_SOURCE
-D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64   -Os  -Wall -Wcast-align
-Wshadow -Wstrict-prototypes -Wno-pointer-sign -c pinentry-curses.c
mips64el-ctng_n64-linux-gnu-gcc: ERROR: unsafe header/library path used
in cross-compilation: '/usr/include/ncursesw'
make[3]: *** [pinentry-curses.o] Error 1

Fixes:

  http://autobuild.buildroot.net/results/5b9/5b9b53a37f7cf4bab4d989f852726a0f0885605f/



[Peter: use --with-ncurses-include-dir=none instead]
Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>