webrtc: Fix make distcheck for g-i build

ERROR: ../../../../../gst-libs/gst/webrtc/webrtc-enumtypes.c: no such a file or directory
diff --git a/gst-libs/gst/webrtc/Makefile.am b/gst-libs/gst/webrtc/Makefile.am
index 95b444e..9ee4291 100644
--- a/gst-libs/gst/webrtc/Makefile.am
+++ b/gst-libs/gst/webrtc/Makefile.am
@@ -57,7 +57,7 @@
 BUILT_GIRSOURCES = GstWebRTC-@GST_API_VERSION@.gir
 
 gir_headers=$(patsubst %,$(srcdir)/%, $(libgstwebrtc_@GST_API_VERSION@include_HEADERS))
-gir_sources=$(patsubst %,$(srcdir)/%, $(libgstwebrtc_@GST_API_VERSION@_la_SOURCES) $(BUILT_SOURCES))
+gir_sources=$(patsubst %,$(srcdir)/%, $(libgstwebrtc_@GST_API_VERSION@_la_SOURCES)) $(BUILT_SOURCES)
 
 GstWebRTC-@GST_API_VERSION@.gir: $(INTROSPECTION_SCANNER) libgstwebrtc-@GST_API_VERSION@.la
 	$(AM_V_GEN)GST_PLUGIN_SYSTEM_PATH_1_0="" GST_PLUGIN_PATH_1_0="" GST_REGISTRY_UPDATE=no GI_SCANNER_DISABLE_CACHE=yes \