[libc] Add the src/stdlib/_Exit.h header to at_quick_exit_test.cpp and atexit_test.cpp (#115351)

Hello, I merged this #114904 a
few mins ago and the tests failed because i did not add the header
`src/stdlib/_Exit.h` in `at_quick_exit_test.cpp` and `atexit_test.cpp`.

I ran both builds/tests and everything was good. thanks

Author: Job Henandez Lara

libc/test/src/stdlib/at_quick_exit_test.cpp

@@ -8,6 +8,7 @@
 
 #include "src/__support/CPP/array.h"
 #include "src/__support/CPP/utility.h"
+#include "src/stdlib/_Exit.h"
 #include "src/stdlib/at_quick_exit.h"
 #include "src/stdlib/quick_exit.h"
 #include "test/UnitTest/Test.h"

libc/test/src/stdlib/atexit_test.cpp

@@ -8,6 +8,7 @@
 
 #include "src/__support/CPP/array.h"
 #include "src/__support/CPP/utility.h"
+#include "src/stdlib/_Exit.h"
 #include "src/stdlib/atexit.h"
 #include "src/stdlib/exit.h"
 #include "test/UnitTest/Test.h"