selftests: vm: Fix test build failure when built by itself

selftests: vm: Fix test build failure when built by itself

vm test build fails when test is built by itself using

make -C tools/testing/selftests/vm
or
cd tools/testing/selftests/vm; make

When the test is built invoking its Makefile directly, it defines
OUTPUT which conflicts with lib.mk's logic to install headers.

make --no-builtin-rules INSTALL_HDR_PATH=$OUTPUT/usr \
        ARCH=x86 -C ../../../.. headers_install
make[1]: Entering directory '/mnt/data/lkml/linux_5.2'
  REMOVE  shmparam.h
rm: cannot remove '/usr/include/asm-generic/shmparam.h': Permission denied
scripts/Makefile.headersinst:96: recipe for target '/usr/include/asm-generic/.install' failed
make[3]: *** [/usr/include/asm-generic/.install] Error 1
scripts/Makefile.headersinst:32: recipe for target 'asm-generic' failed
make[2]: *** [asm-generic] Error 2
Makefile:1199: recipe for target 'headers_install' failed
make[1]: *** [headers_install] Error 2
make[1]: Leaving directory '/mnt/data/lkml/linux_5.2'
../lib.mk:52: recipe for target 'khdr' failed
make: *** [khdr] Error 2

Fixes: 8ce72dc32578 ("selftests: fix headers_install circular dependency")
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>