Add buildfix for kernel 2.6.21

SVN-Revision: 16117

target/linux/generic-2.6/patches-2.6.21/990-buildfix.patch

+--- a/scripts/mod/sumversion.c
++++ b/scripts/mod/sumversion.c
+@@ -1,3 +1,4 @@
++#include <linux/limits.h>
+ #include <netinet/in.h>
+ #ifdef __sun__
+ #include <inttypes.h>