build: move BEES_VERSION to a separate C file to avoid unnecessary building

Every git commit was causing bees.cc and bees-hash.cc to be rebuilt,
which was expensive and unnecessary.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees.h

@@ -819,6 +819,7 @@ public:
 };
 
 // And now, a giant pile of extern declarations
+extern const char *BEES_VERSION;
 string pretty(double d);
 extern RateLimiter bees_info_rate_limit;
 void bees_sync(int fd);