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/rmmod.c b/tools/rmmod.c
index c6f1aee..75fc7b5 100644
--- a/tools/rmmod.c
+++ b/tools/rmmod.c
@@ -28,7 +28,7 @@
 
 #include <shared/macro.h>
 
-#include <libkmod.h>
+#include <libkmod/libkmod.h>
 
 #include "kmod.h"