build: add -Wno-format-signedness

build: add -Wno-format-signedness

gcc5 introduced this option (gcc4 silently ignores it, which is fine).
Given that gcc5 thinks 'unsigned char'/'unsigned short' is promoted to
'int' for var-args, stuff like this spits out warnings:
    uint8_t x;
    printf("%" PRIu8", x);

gcc5 promots 'x' to 'int', instead of 'unsigned int' and thus gets a
signedness-warnings as it expects an 'unsigned int'.

glibc states otherwise: unsigneds are always promoted to 'unsigned int'.
Until gcc and glibc figure this out, lets just ignore that warning (which
is totally useless in its current form).