selftests: gpio: fix uninitialised variable warning

When compiled with -Wall gpio-mockup-cdev.c reports an uninitialised
variable warning.  This is a false positive, as the variable is ignored
in the case it is uninitialised, but initialise the variable anyway
to remove the warning.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>

tools/testing/selftests/gpio/gpio-mockup-cdev.c

@@ -117,7 +117,7 @@ int main(int argc, char *argv[])
 {
 	char *chip;
 	int opt, ret, cfd, lfd;
-	unsigned int offset, val, abiv;
+	unsigned int offset, val = 0, abiv;
 	uint32_t flags_v1;
 	uint64_t flags_v2;