[libc] fix build failures due to SIGCHLD (#94093)

Author: SchrodingerZhu

libc/test/src/__support/threads/linux/CMakeLists.txt

@@ -9,4 +9,5 @@ add_libc_test(
     libc.src.sys.mman.mmap
     libc.src.sys.mman.munmap
     libc.src.stdlib.exit
+    libc.hdr.signal_macros
 )

libc/test/src/__support/threads/linux/raw_mutex_test.cpp

@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "hdr/signal_macros.h"
 #include "include/llvm-libc-macros/linux/time-macros.h"
 #include "src/__support/CPP/atomic.h"
 #include "src/__support/OSUtil/syscall.h"