Add validation caching interface.

src/trusted/service_runtime/sel_ldr.c

Index: src/trusted/service_runtime/sel_ldr.c
diff --git a/src/trusted/service_runtime/sel_ldr.c b/src/trusted/service_runtime/sel_ldr.c
index 9fa58f4f648ea07b3612553dd129eeebfa571ef6..66bc1e32c97f0564335a59a840fbf08cb8be2786 100644
--- a/src/trusted/service_runtime/sel_ldr.c
+++ b/src/trusted/service_runtime/sel_ldr.c
@@ -79,6 +79,9 @@ int NaClAppWithSyscallTableCtor(struct NaClApp               *nap,
   /* Get the set of features that the CPU we're running on supports. */
   NaClGetCurrentCPUFeatures(&nap->cpu_features);
 
+  /* The validation cache will be injected later, if it exists. */
+  nap->validation_cache = NULL;
+
   nap->addr_bits = NACL_MAX_ADDR_BITS;
 
   nap->stack_size = NACL_DEFAULT_STACK_MAX;