include time.h, to prevent use of struct timespec before definition

In some old (2.6.32.x) kernel headers, asound.h does not include linux time.h
when __KERNEL__ is undefined, which may break userspace.

The build failures caused by this are similar to those fixed by c8333f8c.

Signed-off-by: Dima Krasner <dima@dimakrasner.com>
diff --git a/mixer.c b/mixer.c
index 7c5c5dc..9dd6e68 100644
--- a/mixer.c
+++ b/mixer.c
@@ -34,6 +34,7 @@
 #include <errno.h>
 #include <ctype.h>
 #include <limits.h>
+#include <time.h>
 
 #include <sys/ioctl.h>