src/include/pcp/pmapi.h: add alloc debug flag
src/include/pcp/pmapi.h: add alloc debug flag

Current use is just to diagnose unusual mem leak in the derived metric
pmFetch() to pmFreeResult() code path.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp