CLEANUP: 51d: move the 51d dummy lib to contrib/51d/src to match the real lib

This way the directory structure remains the same as with the real lib and
one can apply the same build options regardless of where the lib is stored,
removing any possible confusion.

doc/51Degrees-device-detection.txt

@@ -58,9 +58,11 @@ For HAProxy developers who need to verify that their changes didn't affect the
 directory. This does not function, but implements the API such that the
 51Degrees module can be used (but not return any meaningful information). To
 test either Pattern or Hash Trie, build with:
-    $ make TARGET=<target> USE_51DEGREES=1 51DEGREES_SRC=contrib/51d/pattern
+
+    $ make TARGET=<target> USE_51DEGREES=1 51DEGREES_SRC=contrib/51d/src/pattern
 or
-    $ make TARGET=<target> USE_51DEGREES=1 51DEGREES_SRC=contrib/51d/trie
+    $ make TARGET=<target> USE_51DEGREES=1 51DEGREES_SRC=contrib/51d/src/trie
+
 respectively.
 
 The configuration file needs to set the following parameters: