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