fix algorithm header include

The file is using `std::find_if` without specifying the `algorithm`
include. It has probably transitively included it previously.

Without it, the redpanda open source build fails with the following:
```
/var/lib/buildkite-agent/builds/buildkite-amd64-builders-i-02d9cb4eee45a1cf5-1/redpanda/redpanda/build/release-ci/_deps/avro-src/lang/c++/impl/Node.cc:112:73: error: no member named 'find_if' in namespace 'std'; did you mean 'std::ranges::find_if'?
  112 |     if (!ns_.empty() && (ns_[0] == '.' || ns_[ns_.size() - 1] == '.' || std::find_if(ns_.begin(), ns_.end(), invalidChar1) != ns_.end())) {
      |                                                                         ^~~~~~~~~~~~
      |                                                                         std::ranges::find_if
```

lang/c++/impl/Node.cc

@@ -16,6 +16,7 @@
  * limitations under the License.
  */
 
+#include <algorithm>
 #include <cmath>
 #include <unordered_set>
 

lang/c++/impl/parsing/Symbol.hh

@@ -19,6 +19,7 @@
 #ifndef avro_parsing_Symbol_hh__
 #define avro_parsing_Symbol_hh__
 
+#include <algorithm>
 #include <map>
 #include <set>
 #include <sstream>