datasets: fix compile warnings

datasets-string.c:53:20: error: implicit conversion loses integer precision: 'unsigned long' to 'int' [-Werror,-Wshorten-64-to-32]
        return len + 2;
        ~~~~~~ ~~~~^~~
1 error generated.

(cherry picked from commit 844f6011b1)

src/datasets-string.c

@@ -50,7 +50,7 @@ int StringAsBase64(const void *s, char *out, size_t out_size)
     unsigned long len = SCBase64EncodeBufferSize(str->len);
     if (len + 2 > out_size) {
         // linefeed and final zero : signal we need more space
-        return len + 2;
+        return (int)len + 2;
     }
     if (SCBase64Encode((unsigned char *)str->ptr, str->len, (uint8_t *)out, &len) != SC_BASE64_OK)
         return 0;