crt: testcases: Silence a Clang 23 warning about unused-but-set-global

This fixes the following warning (that is treated as an error
in CI):

    ../../testcases/t_sigfpe.c:154:14: warning: variable 'main_threadid' set but not used [-Wunused-but-set-global]
      154 | static DWORD main_threadid;
          |              ^

mingw-w64-crt/testcases/t_sigfpe.c

diff --git a/mingw-w64-crt/testcases/t_sigfpe.c b/mingw-w64-crt/testcases/t_sigfpe.c
index a77b5ee..6cb7263 100644
--- a/mingw-w64-crt/testcases/t_sigfpe.c
+++ b/mingw-w64-crt/testcases/t_sigfpe.c
@@ -1218,6 +1218,7 @@
 
 #if defined(_WIN32)
   main_threadid = GetCurrentThreadId();
+  (void) main_threadid;
 #endif
 
 #ifdef _WIN32