Fix a dependency: sha256.c influences crypto.o, not crypto.c

changes/bug6778

@@ -1,3 +1,4 @@
   o Minor bugfixes:
     - Avoid a warning when building common_sha1.i out of tree. Fixes bug
       6778; bugfix on 0.2.4.1-alpha.
+    - Fix a mis-declared dependency on src/common/crypto.c.

src/common/include.am

@@ -72,4 +72,4 @@ src/common/common_sha1.i: $(libor_SOURCES) $(libor_crypto_a_SOURCES) $(COMMONHEA
 	fi
 
 src/common/util_codedigest.o: src/common/common_sha1.i
-src/common/crypto.c: src/common/sha256.c
+src/common/crypto.o: src/common/sha256.c