Fix includes after change to build-sys

Make the includes be libkmod/libkmod.h for code outside of library. This
fixes the broken build after 1315123 ('build-sys: Don't add libkmod
subdirectory to include path').
diff --git a/tools/kmod.c b/tools/kmod.c
index 4239027..abb1707 100644
--- a/tools/kmod.c
+++ b/tools/kmod.c
@@ -23,7 +23,7 @@
 #include <stdlib.h>
 #include <string.h>
 
-#include <libkmod.h>
+#include <libkmod/libkmod.h>
 
 #include "kmod.h"