ipmid: user_channel: fix include path for libuserlayer

When built as a subdirectory, building in place works, but building with
the source separate from the artifacts (a la Yocto) requires that the
include path is set correctly.

Change-Id: I0558814190e47b64b4bc504eed44d4a67ecdf156
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>
diff --git a/user_channel/Makefile.am b/user_channel/Makefile.am
index 0b62ada..58a652f 100644
--- a/user_channel/Makefile.am
+++ b/user_channel/Makefile.am
@@ -14,6 +14,7 @@
 	$(CRYPTO_LIBS) \
 	-version-info 0:0:0 -shared
 libuserlayer_la_CXXFLAGS = \
+	-I$(top_srcdir) \
 	$(SYSTEMD_CFLAGS) \
 	$(libmapper_CFLAGS) \
 	$(PHOSPHOR_LOGGING_CFLAGS) \