npth_sigmask and npth_sigwait are not symbols, so
-Wl,--no-undefined-version will report errors. Actually npth_sigwait is
not defined, so just remove it. The npth_sigmask macro is used by
gnupg/g13/g13.c, so keep it.
- src/npth.h.in: Remove npth_sigmask and npth_sigwait.
- w32/npth.h: Likewise.
- src/libnpth.vers: Likewise.
symbols to fix -Wl,--no-undefined-version