src/Makefile: build util.c separately for makeguids
src/Makefile: build util.c separately for makeguids

util.c needs to be built twice when cross-compiling:
for the build machine to be able to link with
makeguids which then runs during the same build,
and then for the actual target.

Signed-off-by: Alexander Kanavin <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@linutronix.de>

rhboot/efivar,rhboot/efivar