core: add missing system includes

for:
open(2), close(2), etc.
for types like ssize_t

src/nm-manager.c

@@ -25,6 +25,8 @@
 #include <fcntl.h>
 #include <errno.h>
 #include <string.h>
+#include <unistd.h>
+#include <sys/types.h>
 #include <sys/stat.h>
 #include <dbus/dbus-glib-lowlevel.h>
 #include <dbus/dbus-glib.h>