config: fix compilation with FLB_HAVE_STATIC_CONF

Fixes:

```
/tmp/fluent-bit/src/config_format/flb_cf_fluentbit.c: In function ‘read_config’:
/tmp/fluent-bit/src/config_format/flb_cf_fluentbit.c:505:30: error: ‘FLB_CF_BUF_SIZE’ undeclared (first use in this function); did you mean ‘FLB_CF_CUSTOM’?
     while (static_fgets(buf, FLB_CF_BUF_SIZE, in_data, &off)) {
                              ^~~~~~~~~~~~~~~
```

This applies the same change made in
f50a02e when `FLB_HAVE_STATIC_CONF` is
defined.

src/config_format/flb_cf_fluentbit.c

@@ -502,7 +502,7 @@ static int read_config(struct flb_cf *cf, struct local_ctx *ctx,
      * workaround to retrieve the lines.
      */
     size_t off = 0;
-    while (static_fgets(buf, FLB_CF_BUF_SIZE, in_data, &off)) {
+    while (static_fgets(buf, bufsize - (fgets_ptr - buf), in_data, &off)) {
 #else
     /* normal mode, read lines into a buffer */
     /* note that we use "fgets_ptr" so we can continue reading after realloc */