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 <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Reviewed-by: Quentin Monnet <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/bpftool/json_writer.c

···
 			fputs("\\b", self->out);
 			break;
 		case '\\':
-			fputs("\\n", self->out);
+			fputs("\\\\", self->out);
 			break;
 		case '"':
 			fputs("\\\"", self->out);

tools/bpf/bpftool/json_writer.h

···
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdarg.h>
+#include <stdio.h>
 #include <linux/compiler.h>
 
 /* Opaque class structure */