Eliminate extern-initializer warning

[danielytics]

When including crow.h (or, rather, routing.h) without CROW_MAIN, the INVALID_BP_ID variable is declared as extern, which is triggers the -Wextern-initializer warning (at least with clang):

include/crow/routing.h:25:21: error: 'extern' variable has an initializer [-Werror,-Wextern-initializer]                                                                                                                           
    extern uint16_t INVALID_BP_ID{0xFFFF};   

Since Crow is header-only, this means that any sources using Crow cannot be compiled with -Werror or similar. This PR simply removes the initialzer when declaring as extern.

[The-EDev]

Thanks for noticing this and fixing it, it was an error on my part. Merging now