Fix type mismatch in mutt_pretty_size()

Fix type mismatch in mutt_pretty_size()

Fixes the following GCC warnings:

    muttlib.c: In function ‘mutt_pretty_size’:
    config.h:55:19: warning: format ‘%lld’ expects argument of type ‘long long int’, but argument 4 has type ‘size_t {aka unsigned int}’ [-Wformat=]
     #define OFF_T_FMT "%" PRId64
                       ^
    muttlib.c:614:22: note: in expansion of macro ‘OFF_T_FMT’
         snprintf(s, len, OFF_T_FMT "K", (n + 51) / 1024);
                          ^~~~~~~~~
    config.h:55:19: warning: format ‘%lld’ expects argument of type ‘long long int’, but argument 4 has type ‘size_t {aka unsigned int}’ [-Wformat=]
     #define OFF_T_FMT "%" PRId64
                       ^
    muttlib.c:621:22: note: in expansion of macro ‘OFF_T_FMT’
         snprintf(s, len, OFF_T_FMT "M", (n + 52428) / 1048576);
                          ^~~~~~~~~