src: make base64.h self-contained

This commit includes stdint.h (for uint8_t, uint32_t, and int8_t) to
make it self-contained.

PR-URL: #17177
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>

src/base64.h

@@ -6,6 +6,7 @@
 #include "util.h"
 
 #include <stddef.h>
+#include <stdint.h>
 
 namespace node {
 //// Base 64 ////