Add IOV_MAX to <limits.h>.

strace 4.13 assumes we have IOV_MAX. POSIX says it lives here. 1024 is
a Linux kernel limitation.

Change-Id: Icb428172670e4936a459db38a6a190b7178fe9f3
diff --git a/libc/include/limits.h b/libc/include/limits.h
index cb86aec..a1124b3 100644
--- a/libc/include/limits.h
+++ b/libc/include/limits.h
@@ -82,6 +82,7 @@
 
 #define MB_LEN_MAX 4
 
+#define IOV_MAX 1024
 #define SEM_VALUE_MAX 0x3fffffff
 
 /* POSIX says these belong in <unistd.h> but BSD has some in <limits.h>. */