tools: bpftool: json: Fix backslash escape typo in jsonw_puts

This is essentially a backport of iproute2's
commit ed54f76484b5 ("json: fix backslash escape typo in jsonw_puts")

Also added the stdio.h include in json_writer.h to be able to compile
and run the json_writer test as used below).

Before this fix:

$ gcc -D notused -D TEST -I../../include -o json_writer  json_writer.c
json_writer.h
$ ./json_writer
{
    "Vyatta": {
        "url": "http://vyatta.com",
        "downloads": 2000000,
        "stock": 8.16,
        "ARGV": [],
        "empty": [],
        "NIL": {},
        "my_null": null,
        "special chars": [
            "slash": "/",
            "newline": "\n",
            "tab": "\t",
            "ff": "\f",
            "quote": "\"",
            "tick": "'",
            "backslash": "\n"
        ]
    }
}

After:

$ gcc -D notused -D TEST -I../../include -o json_writer  json_writer.c
json_writer.h
$ ./json_writer
{
    "Vyatta": {
        "url": "http://vyatta.com",
        "downloads": 2000000,
        "stock": 8.16,
        "ARGV": [],
        "empty": [],
        "NIL": {},
        "my_null": null,
        "special chars": [
            "slash": "/",
            "newline": "\n",
            "tab": "\t",
            "ff": "\f",
            "quote": "\"",
            "tick": "'",
            "backslash": "\\"
        ]
    }
}
Signed-off-by: Manu Bretelle <chantr4@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20230329073002.2026563-1-chantr4@gmail.com

tools/bpf/bpftool/json_writer.c

@@ -75,7 +75,7 @@ static void jsonw_puts(json_writer_t *self, const char *str)
 			fputs("\\b", self->out);
 			break;
 		case '\\':
-			fputs("\\n", self->out);
+			fputs("\\\\", self->out);
 			break;
 		case '"':
 			fputs("\\\"", self->out);

tools/bpf/bpftool/json_writer.h

@@ -14,6 +14,7 @@
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdarg.h>
+#include <stdio.h>
 #include <linux/compiler.h>
 
 /* Opaque class structure */