Polyspace provides stubs for all standard library function like memcpy. If your implementation differs from that standard implementation then you might receive an error message like the one mentioned above.
In that case you can switch off the standard library stubbing for this particular function by adding the define
-D __polyspace_no_memcpy
to the analysis options. So Polyspace would either use your implementation of memcpy or it would stub the function based on the prototype you mention in the header file.
For details see page 7-53 (Linking: Conflict with Standard Library Function Stubs) in the Polyspace Code Prover User's Guide, which can be found online at
https://www.mathworks.com/help/pdf_doc/codeprover/codeprover_ug.pdf
