Add install-headers target

Install the libopenbmc_intf includes in prep for a devel
package.

Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/rules.mk b/rules.mk
index fb6c000..c7075c1 100644
--- a/rules.mk
+++ b/rules.mk
@@ -3,6 +3,7 @@
 .DEFAULT_GOAL := all
 sbindir=/usr/sbin
 libdir=/usr/lib
+includedir=/usr/include
 
 LDLIBS+=$(shell pkg-config --libs $(PACKAGE_DEPS))
 ALL_CFLAGS+=$(shell pkg-config --cflags $(PACKAGE_DEPS)) -fPIC -Werror $(CFLAGS)