Fix a misspelled macro test that was breaking big-endian OPE

Fixes bug 28115; bugfix on 0.3.5.1-alpha.

changes/bug28115

+  o Minor bugfixes (portability):
+    - Make the OPE code (which is used for v3 onion services) run correctly
+      on big-endian platforms. Fixes bug 28115; bugfix on 0.3.5.1-alpha.

src/lib/crypt_ops/crypto_ope.c

@@ -48,7 +48,7 @@ struct crypto_ope_t {
 /** The type to add up in order to produce our OPE ciphertexts */
 typedef uint16_t ope_val_t;
 
-#ifdef WORDS_BIG_ENDIAN
+#ifdef WORDS_BIGENDIAN
 /** Convert an OPE value to little-endian */
 static inline ope_val_t
 ope_val_to_le(ope_val_t x)