unix: fix implicit declaration compiler warning

Include <string.h> explicitly to get the definition of strncpy().

Refs https://github.com/libuv/libuv/issues/138.

PR-URL: https://github.com/libuv/libuv/pull/150
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

src/unix/darwin-proctitle.c

@@ -21,6 +21,7 @@
 #include <dlfcn.h>
 #include <errno.h>
 #include <stdlib.h>
+#include <string.h>
 
 #include <TargetConditionals.h>