selftests: Add missing #include directives

Several C programs fail to include the headers declaring all the
functions they call, resulting in warnings or errors.

After this, memfd_test.c is still missing some function declarations
but can't easily get them because of a conflict between
<linux/fcntl.h> and <sys/fcntl.h>.

Signed-off-by: Ben Hutchings <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/memfd/memfd_test.c

···
 #include <sys/mman.h>
 #include <sys/stat.h>
 #include <sys/syscall.h>
+#include <sys/wait.h>
 #include <unistd.h>
 
 #define MFD_DEF_SIZE 8192

tools/testing/selftests/mqueue/mq_open_tests.c

···
 #include <sys/resource.h>
 #include <sys/stat.h>
 #include <mqueue.h>
+#include <error.h>
 
 static char *usage =
 "Usage:\n"

tools/testing/selftests/mqueue/mq_perf_tests.c

···
 #include <sys/stat.h>
 #include <mqueue.h>
 #include <popt.h>
+#include <error.h>
 
 static char *usage =
 "Usage:\n"

tools/testing/selftests/timers/nanosleep.c

···
  *   GNU General Public License for more details.
  */
 
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>