heap: create unique variable name

In code is a variable "chunksz_t chunksz" that has the same name as
function "chunksz_t chunksz()" in the one heap.h file.
Create unique variable name to avoid misreading in the future.

Found as a coding guideline violation (MISRA R5.9) by static
coding scanning tool.

Signed-off-by: Maksim Masalski <maksim.masalski@intel.com>