LibC: Implement __freadahead

Author: https://github.com/timschumi
Commit: 89ed0649f7
Pull-request: https://github.com/SerenityOS/serenity/pull/13333

Userland/Libraries/LibC/stdio.cpp

@@ -1328,6 +1328,17 @@ void __fpurge(FILE* stream)
     stream->purge();
 }
 
+size_t __freadahead(FILE* stream)
+{
+    VERIFY(stream);
+
+    ScopedFileLock lock(stream);
+
+    size_t available_size;
+    stream->readptr(available_size);
+    return available_size;
+}
+
 char const* __freadptr(FILE* stream, size_t* sizep)
 {
     VERIFY(stream);