Add LIBZIP_CFLAGS in libraries/amf/Makefile.am to fix FTBFS on Debian.

Currently Debian ships libzip 0.11.2. Due to multi-arch support, zipconf.h
is inside /usr/lib/<arch>/libzip/include and we must use LIBZIP_CFLAGS
which comes from pkg-config to avoid the build failed.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>
Merge pull request #111 from grandpaul/fix-amf-ftbfs

Add LIBZIP_CFLAGS in libraries/amf/Makefile.am to fix FTBFS on Debian.