Drop <sys/unistd.h> include

<sys/unistd.h> does not exist in musl or uClibc, so including it
causes a build failure. In glibc, it simply redirects to <unistd.h>,
so we can safely drop the inclusion of <sys/unistd.h>

Signed-off-by: Christopher McCrory <chrismcc@gmail.com>

Index: b/CPU.xs
===================================================================
--- a/CPU.xs
+++ b/CPU.xs
@@ -24,7 +24,6 @@
  #include <winreg.h>
 #else                /* other (try unix) */
  #include <unistd.h>
- #include <sys/unistd.h>
 #endif
 #if defined(__sun) || defined(__sun__) 
  #include <sys/processor.h>