autogen.sh: adopt cleaning after API change
autogen.sh: adopt cleaning after API change

The pkg-config file is now named libcoap-1.pc and was forgotten to
change too in autogen.sh while changing the API versioning within the
commits 1ea9e1c and 24a83b7.

authmillenon/libcoap,gebart/libcoap,authmillenon/libcoap,authmillenon/libcoap,gebart/libcoap,gebart/libcoap