Move include directive from CFLAGS to LOCAL_CPPFLAGS in examples' Makefiles

The use of LOCAL_* flags and override directives ensures that the build
succeeds even if the user explicitly overrides CFLAGS/CPPFLAGS/LDFLAGS.

Fixes #537

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>