shared/time-util: fix gcc5 warning

CC       src/shared/libsystemd_shared_la-time-util.lo
src/shared/time-util.c: In function 'parse_nsec':
src/shared/time-util.c:789:25: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
                 if (!*s != 0)
                         ^

src/shared/time-util.c

@@ -786,7 +786,7 @@ int parse_nsec(const char *t, nsec_t *nsec) {
         s = startswith(p, "infinity");
         if (s) {
                 s += strspn(s, WHITESPACE);
-                if (!*s != 0)
+                if (*s != 0)
                         return -EINVAL;
 
                 *nsec = NSEC_INFINITY;