firmware-utils: mkfwimage: fix more errors reported by gcc-6/7/9

firmware-utils: mkfwimage: fix more errors reported by gcc-6/7/9

src/mkfwimage.c:276:8: error: format '%lld' expects argument of type 'long long int', but argument 4 has type '__off_t' {aka 'const long int'} [-Werror=format=]
src/fw.h:71:36: error: format '%llu' expects argument of type 'long long unsigned int', but argument 6 has type '__off_t' {aka 'long int'} [-Werror=format=]
    inlined from 'main' at src/mkfwimage.c:543:12:
/string_fortified.h:106:10: error: '__builtin_strncpy' output truncated before terminating nul copying 4 bytes from a string of the same length [-Werror=stringop-truncation]
    inlined from 'write_part' at src/mkfwimage.c:235:2,
string_fortified.h:106:10: error: '__builtin_strncpy' specified bound 16 equals destination size [-Werror=stringop-truncation]
    inlined from 'main' at src/mkfwimage.c:477:5:
string_fortified.h:106:10: error: '__builtin_strncpy' specified bound 256 equals destination size [-Werror=stringop-truncation]
    inlined from 'main' at src/mkfwimage.c:496:5:
string_fortified.h:106:10: error: '__builtin_strncpy' specified bound 4096 equals destination size [-Werror=stringop-truncation]
    inlined from 'main' at src/mkfwimage.c:481:5:
string_fortified.h:106:10: error: '__builtin_strncpy' specified bound 4096 equals destination size [-Werror=stringop-truncation]
    inlined from 'main' at src/mkfwimage.c:485:5:
string_fortified.h:106:10: error: '__builtin_strncpy' specified bound 16 equals destination size [-Werror=stringop-truncation]

Runtested on ath79 and UBNT Bullet M XW.

Signed-off-by: Petr Štetiar <ynezz@true.cz>