selftests/futex: Fix line continuation in Makefile

The Makefile lacks a couple of line continuation backslashes
in an `if' clause, which produces an error when make versions
prior to 4.x are used for building the tests.

  $ make
  make[1]: Entering directory `/[...]/linux/tools/testing/selftests/futex'
  /bin/sh: -c: line 5: syntax error: unexpected end of file
  make[1]: *** [all] Error 1
  make[1]: Leaving directory `/[...]/linux/tools/testing/selftests/futex'
  make: *** [all] Error 2

Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>