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 <ben@decadent.org.uk>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/memfd/memfd_test.c

@@ -15,6 +15,7 @@
 #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

@@ -31,6 +31,7 @@
 #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

@@ -37,6 +37,7 @@
 #include <sys/stat.h>
 #include <mqueue.h>
 #include <popt.h>
+#include <error.h>
 
 static char *usage =
 "Usage:\n"

tools/testing/selftests/timers/nanosleep.c

@@ -19,6 +19,7 @@
  *   GNU General Public License for more details.
  */
 
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>