Use machine/timespec.h to separate timespec definition.

Bug: 18627763
Change-Id: Ib1c406320f0ecc3705d19183a48f986cdf57c279
diff --git a/libc/include/signal.h b/libc/include/signal.h
index 867f497..9519f6d 100644
--- a/libc/include/signal.h
+++ b/libc/include/signal.h
@@ -32,8 +32,8 @@
 #include <asm/sigcontext.h>
 #include <errno.h>
 #include <limits.h>
-#include <linux/time.h>
 #include <machine/pthread_types.h>
+#include <machine/timespec.h>
 #include <string.h>
 #include <sys/cdefs.h>
 #include <sys/types.h>