CONTRIB: spoa_example: allow to compile outside HAProxy.

Don't include haproxy's includes anymore and use a local copy instead.
diff --git a/contrib/spoa_example/Makefile b/contrib/spoa_example/Makefile
index 3105ef0..d04a01e 100644
--- a/contrib/spoa_example/Makefile
+++ b/contrib/spoa_example/Makefile
@@ -6,8 +6,8 @@
 LD = $(CC)
 
 CFLAGS  = -g -O2 -Wall -Werror -pthread
-LDFLAGS = -lpthread  -levent -levent_pthreads
-INCS += -I../../include -I../../ebtree
+LDFLAGS = -lpthread -levent -levent_pthreads
+INCS += -I../../ebtree -I./include
 LIBS =
 
 OBJS = spoa.o