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>