rtc.c test: fix for clang

clang complains:
> clang -g -O2 -Wall -Werror -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=0  -iquote ../lib/arch/x86/include -I../lib   rtc.c ../lib/libzdtmtst.a ../lib/libzdtmtst.a -o rtc
> rtc.c:50:7: error: taking the absolute value of unsigned type 'unsigned long' has no effect [-Werror,-Wabsolute-value]
>                 if (labs(delta - 1000000 / TEST_HZ ) > 100000) {
>                     ^

Declare delta as long.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

test/zdtm/static/rtc.c

@@ -15,7 +15,8 @@
 
 int main(int argc, char **argv)
 {
-	unsigned long data, delta;
+	unsigned long data;
+	long delta;
 	int fd, fail = NR_FAILS, to_pass = NR_FAILS;
 	struct timeval start, end;