cpupower: Disable direct build of the 'bench' subproject

cpupower: Disable direct build of the 'bench' subproject

Execution of the 'make' command in the 'bench' subfolder causes the
following error:

$ make O=cpupower/build/ DESTDIR=cpupower/install/ -j8
"  CC      " cpupower/build//main.o
"  CC      " cpupower/build//parse.o
/bin/sh: 1: "  CC      "cpupower/build//system.o
  CC      : not found
  make: *** [Makefile:21: cpupower/build//main.o] Error 127
  make: *** Waiting for unfinished jobs....
  /bin/sh: 1:   CC      : not found
  /bin/sh: 1:   CC      : not found
  make: *** [Makefile:21: cpupower/build//parse.o] Error 127
  make: *** [Makefile:21: cpupower/build//system.o] Error 127

The makefile uses variables defined in the main project makefile and it
is not intended to run standalone. The reason is that 'bench' subproject
depends on the 'libcpupower' library, see the 'compile-bench' target in
the main makefile.
Add a check that prevents standalone execution of the 'bench' makefile.

Signed-off-by: Roman Storozhenko <romeusmeister@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>