uimage_sgehdr: use "char" type for header struct strings

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>