Fix trivial typo in kex.c

Change "cppstr" to "ccpstr", because it's not chaPHA20-poly1305, after
all.

kex.c

@@ -809,7 +809,7 @@ patch_list(char * orig)
  adj = xreallocarray(adj,
  strlen(adj) /* original string length */
  + 1 /* for the original null-terminator */
- + strlen(cppstr) /* make room for ccpstr */
+ + strlen(ccpstr) /* make room for ccpstr */
  + 1 /* make room for the comma delimiter */
  , sizeof(char));
  /*