Fix C flag declaration for Dune 2.7.0

Otherwise, the string "-I ." is directly passed to `gcc` which makes it
ignore the flag.

Author: bbc2

src/dune

@@ -1,7 +1,7 @@
 (library
   (public_name zxcvbn)
   (c_names zxcvbn_stubs zxcvbn)
-  (c_flags (:standard "-I ."))
+  (c_flags (:standard -I .))
   (preprocess
     (pps
       ppx_deriving.std