uimage_sgehdr: use "char" type for header struct strings
This fixes:
src/uimage_sgehdr.c: In function ‘main’:
src/uimage_sgehdr.c:152:10: warning: pointer targets in passing argument 1 of ‘strncpy’ differ in signedness [-Wpointer-sign]
strncpy(imgh->sgeih_p, model, sizeof(imgh->sgeih_p));
^~~~
src/uimage_sgehdr.c:153:10: warning: pointer targets in passing argument 1 of ‘strncpy’ differ in signedness [-Wpointer-sign]
strncpy(imgh->sgeih_sv, sversion, sizeof(imgh->sgeih_sv));
^~~~
src/uimage_sgehdr.c:154:10: warning: pointer targets in passing argument 1 of ‘strncpy’ differ in signedness [-Wpointer-sign]
strncpy(imgh->sgeih_hv, hversion, sizeof(imgh->sgeih_hv));
^~~~
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>