learned_dscp is a uint8_t and not a unsigned short, pass u to printf ins...

Hi
libpcp doesn't compile with clang without this fix. I get:

    pcp_app.c:362:18: error: format specifies type 'unsigned short' but the argument has type 'uint8_t' (aka 'unsigned char') [-Werror,-Wformat]
                     ret->learned_dscp,
                 ^~~~~~~~~~~~~~~~~
    1 error generated.

I have successfully compiled with both clang and gcc after modifying the code. All tests pass as well.