libkmod: fix integration with gtk-doc

It was failing to generate doc with recent version of gtk-doc

[kmod]$ ./bootstrap
libkmod/docs/gtk-doc.make:33: error: EXTRA_DIST must be set with '=' before using '+='
libkmod/docs/Makefile.am:29:   'libkmod/docs/gtk-doc.make' included from here
autoreconf: automake failed with exit status: 1

Just add an empty EXTRA_DIST so it works.
diff --git a/libkmod/docs/Makefile.am b/libkmod/docs/Makefile.am
index 06f65c8..c4f3d69 100644
--- a/libkmod/docs/Makefile.am
+++ b/libkmod/docs/Makefile.am
@@ -25,8 +25,10 @@
 
 content_files = version.xml
 
+EXTRA_DIST =
+
 if ENABLE_GTK_DOC
 include $(top_srcdir)/libkmod/docs/gtk-doc.make
 else
-EXTRA_DIST = libkmod-docs.xml libkmod-sections.txt
+EXTRA_DIST += libkmod-docs.xml libkmod-sections.txt
 endif