darwin: fix ios build error

Include <errno.h> and <stdlib.h>.  They're being pulled in implicitly
on OS X but apparently that's not the case with iOS builds.

Build breakage introduced in 5ff6f85 by yours truly. Mea culpa.

Fixes #885 and #891.

src/unix/darwin-proctitle.c

@@ -18,8 +18,11 @@
  * IN THE SOFTWARE.
  */
 
-#include <TargetConditionals.h>
 #include <dlfcn.h>
+#include <errno.h>
+#include <stdlib.h>
+
+#include <TargetConditionals.h>
 
 #if !TARGET_OS_IPHONE
 # include <CoreFoundation/CoreFoundation.h>