Fix C preprocessor directive

In otherlibs/threads/scheduler.c, an #include directive was used
instead of a #warning or #error one.

This commit replaces it with a #warning directive.

Author: shindere

otherlibs/threads/scheduler.c

@@ -40,7 +40,7 @@
        defined(HAS_SETITIMER) && \
        defined(HAS_GETTIMEOFDAY) && \
        (defined(HAS_WAITPID) || defined(HAS_WAIT4)))
-#include "Cannot compile libthreads, system calls missing"
+#warning "Cannot compile libthreads, system calls missing"
 #endif
 
 #include <errno.h>