RunCMake/CTestTimeout: Include sched.h for pid_t

ISO C modes in Clang don't enable modern POSIX standards.
Include sched.h to define pid_t in such modes.

Tests/RunCMake/CTestTimeout/TestTimeout.c

@@ -1,6 +1,7 @@
 #if defined(_WIN32)
 #  include <windows.h>
 #else
+#  include <sched.h>
 #  include <unistd.h>
 #endif