Cygwin: Add missing Linux #define of CPU_SETSIZE

Though our implementation of cpu sets doesn't need it, software from
Linux environments expects this definition to be present.  It's
documented on the Linux CPU_SET(3) man page but was left out due to
oversight.

Addresses https://cygwin.com/ml/cygwin/2019-12/msg00248.html

winsup/cygwin/include/sys/cpuset.h

@@ -89,6 +89,7 @@ int __sched_getaffinity_sys (pid_t, size_t, cpu_set_t *);
 #define CPU_XOR(dst, src1, src2)  CPU_XOR_S(sizeof (cpu_set_t), dst, src1, src2)
 #define CPU_EQUAL(src1, src2)     CPU_EQUAL_S(sizeof (cpu_set_t), src1, src2)
 
+#define CPU_SETSIZE               __CPU_SETSIZE
 #endif /* __GNU_VISIBLE */
 
 #ifdef __cplusplus

winsup/cygwin/release/3.1.3

@@ -0,0 +1,5 @@
+Bug Fixes
+---------
+
+- Define CPU_SETSIZE, as on Linux.
+  Addresses: https://cygwin.com/ml/cygwin/2019-12/msg00248.html