cmake: fix a cmake error when with -DALLOCATOR=jemalloc.

cmake: fix a cmake error when with -DALLOCATOR=jemalloc.

When exec: ./do_cmake.sh -DALLOCATOR=jemalloc. Met the following
messages:
>> CMake Error at cmake/modules/FindJeMalloc.cmake:28 (endforeach):
  endforeach An ENDFOREACH command was found outside of a proper FOREACH
  ENDFOREACH structure.  Or its arguments did not match the opening FOREACH
  command.
>> Call Stack (most recent call first):
  CMakeLists.txt:326 (find_package)

This bug introduce commit 8db629a14baf2aea99cf.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>