fix: add HaikuOS support for GetTID() in sysinfo.cc

Add a platform-specific GetTID() implementation for HaikuOS using
Haiku's native find_thread(NULL) API, which returns the current
thread's ID. Without this, HaikuOS falls through to the generic
pthread_self() fallback which causes a compilation failure due to
type incompatibility.

Fixes #1981

Author: ShadiBahaa

absl/base/internal/sysinfo.cc

@@ -50,6 +50,10 @@
 #include <zircon/process.h>
 #endif
 
+#if defined(__HAIKU__)
+#include <OS.h>
+#endif
+
 #include <string.h>
 
 #include <cassert>
@@ -466,6 +470,10 @@ pid_t GetTID() {
   return static_cast<pid_t>(zx_thread_self());
 }
 
+#elif defined(__HAIKU__)
+
+pid_t GetTID() { return find_thread(NULL); }
+
 #else
 
 // Fallback implementation of `GetTID` using `pthread_self`.