avutil/cpu: Use HAVE_UNISTD_H instead of HAVE_SYSCONF for #include <unistd.h>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/cpu.c

@@ -40,7 +40,7 @@
 #include <sys/types.h>
 #include <sys/sysctl.h>
 #endif
-#if HAVE_SYSCONF
+#if HAVE_UNISTD_H
 #include <unistd.h>
 #endif