libkmod: Add missing definitions

Depending on kernel header and simply not passing the flags in
finit_module() if this header is not found is not good.

Add a missing.h header in which stuff like this should be added.
diff --git a/Makefile.am b/Makefile.am
index 9feaf96..1680fe6 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -55,6 +55,7 @@
 	libkmod/libkmod.h \
 	libkmod/libkmod-private.h \
 	libkmod/macro.h \
+	libkmod/missing.h \
 	libkmod/libkmod.c \
 	libkmod/libkmod-list.c \
 	libkmod/libkmod-config.c \