Build: configure: explicitly add corosync include path to CPPFLAGS

tests/test-headers.sh compiles with CPPFLAGS but not CFLAGS, but unfortunately
the configure script doesn't add all the include paths to CPPFLAGS.

PKG_CHECK_MODULES doesn't set a cppflags variable, so invoke pkg-config
--cflags-only-I directly to get the include paths for corosync, and add that to
CPPFLAGS. Hopefully that will be sufficient for test-headers, though it's not a
complete solution.