Fix wrong types in format strings used in debug build

Fix wrong types in format strings used in debug build

GCC 9.2.1 on x86/64 complains about following format string issues:

 urngd.c:94:12: error: format ‘%u’ expects argument of type ‘unsigned int’, but argument 3 has type ‘size_t’ {aka ‘long unsigned int’} [-Werror=format=]
 urngd.c:94:12: error: format ‘%u’ expects argument of type ‘unsigned int’, but argument 4 has type ‘size_t’ {aka ‘long unsigned int’} [-Werror=format=]

GCC 7.4.0 on i.mx6 complains about following format string issues:

 urngd.c:55:20: error: format '%lu' expects argument of type 'long unsigned int', but argument 3 has type 'size_t {aka unsigned int}' [-Werror=format=]

Ref: https://gitlab.com/ynezz/openwrt-urngd/-/jobs/306404059
Ref: https://gitlab.com/ynezz/openwrt-urngd/-/jobs/309187631
Signed-off-by: Petr Štetiar <ynezz@true.cz>