elf: Include <stdbool.h> in tst-tls20.c

The test uses the bool type.

elf/tst-tls20.c

@@ -19,6 +19,7 @@
 #include <array_length.h>
 #include <dlfcn.h>
 #include <pthread.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>