Makefile.am: adding libcoap.h to the install targets
Makefile.am: adding libcoap.h to the install targets

After the creation of the new header file libcoap.h we need to add this
file to the installation of libcoap.

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