Fix Cygwin build after 02d04eac

Commit 02d04eac "Use strwinerror in gdb/windows-nat.c" also moves
strwinerror() under the USE_WIN32API conditional, which is not defined
for Cygwin (and looks like it shouldn't be, as appears to imply
non-POSIX and MiNGW and WinSock...)

Also enable the declaration and definition of strwinerror() when
__CYGWIN__ is defined.