losetup: add missing initializer [clang]

losetup: add missing initializer [clang]

sys-utils/losetup.c:290:12: warning: variable 'rc' is used uninitialized
	whenever 'if' condition is false [-Wsometimes-uninitialized]
sys-utils/losetup.c:293:7: note: uninitialized use occurs here
sys-utils/losetup.c:290:8: note: remove the 'if' if its condition is always true
sys-utils/losetup.c:228:9: note: initialize the variable 'rc' to silence
	this warning

Signed-off-by: Sami Kerola <kerolasa@iki.fi>