selftests: futex: Use variable MAKE instead of make

Recursive make commands should always use the variable MAKE, not the
explicit command name ‘make’. This has benefits and removes the
following warning when multiple jobs are used for the build:

make[2]: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.

Fixes: a8ba798b

 ("selftests: enable O and KBUILD_OUTPUT")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>