Build error on Cygwin after randomness merge

[evhan]

Some include headers are missing on Cygwin, causing this error during the build process:

gcc -fno-strict-aliasing -fwrapv -DHAVE_CHICKEN_CONFIG_H -DC_ENABLE_PTABLES -c -Os -fomit-frame-pointer  -DC_BUILDING_LIBCHICKEN runtime.c -o runtime-static.o -I. -I./
runtime.c: In function random_urandom:
runtime.c:12553:10: warning: implicit declaration of function open [-Wimplicit-function-declaration]
     fd = open("/dev/urandom", O_RDONLY);
          ^
runtime.c:12553:31: error: O_RDONLY undeclared (first use in this function)
     fd = open("/dev/urandom", O_RDONLY);
                               ^
runtime.c:12553:31: note: each undeclared identifier is reported only once for each function it appears in
make[1]: *** [rules.make:125: runtime-static.o] Error 1
make[1]: Leaving directory '/home/evhan/chicken-5-cygwin-x86'
make: *** [rules.make:986: boot-chicken] Error 2

Output of uname -a (same happens on x86):

CYGWIN_NT-6.1-WOW windows 2.6.1(0.305/5/3) 2016-12-16 11:50 i686 Cygwin

[sjamaan]

This has been fixed in c04154ee38eca82d8489ae0579c9fede1c040f9c