[glibc] htl: Add missing include

[Samuel Thibault]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=c7d699b55b4e2f5644495a156b0d778105a5e4e3

commit c7d699b55b4e2f5644495a156b0d778105a5e4e3
Author: Samuel Thibault <samuel.thibault@ens-lyon.org>
Date:   Sun Nov 16 11:53:46 2025 +0100

    htl: Add missing include
    
    For IS_IN.

Diff:
---
 sysdeps/htl/pthreadP.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/sysdeps/htl/pthreadP.h b/sysdeps/htl/pthreadP.h
index 8ea60949ec..471a4cfa43 100644
--- a/sysdeps/htl/pthreadP.h
+++ b/sysdeps/htl/pthreadP.h
@@ -24,6 +24,7 @@
 #include <pthread.h>
 #include <link.h>
 #include <bits/cancelation.h>
+#include <libc-symbols.h>
 
 /* Attribute to indicate thread creation was issued from C11 thrd_create.  */
 #define ATTR_C11_THREAD ((void*)(uintptr_t)-1)