firmware_handler: add fake sdbusplus if not found

If HAVE_SDBUSPLUS is set, use the real header, otherwise define a fake
sdbusplus/bus/bus object.

Change-Id: I70e343dd726c2eaf0f03c0ab96898686161ade27
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/configure.ac b/configure.ac
index b722ec1..78c9887 100644
--- a/configure.ac
+++ b/configure.ac
@@ -24,7 +24,7 @@
 PKG_CHECK_MODULES(
     [SDBUSPLUS],
     [sdbusplus],
-    [],
+    [AC_DEFINE(HAVE_SDBUSPLUS, [1], [Found openbmc/sdbusplus package])],
     [AC_MSG_ERROR([The openbmc/sdbusplus package is required])]
 )
 PKG_CHECK_MODULES(