Add mkdtemp() prototype to stdlib.h.

The mkdtemp() function is implemented in libc/stdio/mktemp.c but not
exposed in stdlib.h. This change adds the prototype to stdlib.h.

Change-Id: I5a98650c665d2e45b2cf6ed3382742f7bdc7c88a
diff --git a/libc/include/stdlib.h b/libc/include/stdlib.h
index 97d8e46..5dc8a87 100644
--- a/libc/include/stdlib.h
+++ b/libc/include/stdlib.h
@@ -57,6 +57,7 @@
 extern int unsetenv(const char *);
 extern int clearenv(void);
 
+extern char *mkdtemp(char *);
 extern char *mktemp (char *);
 extern int mkstemp (char *);