lib: add missing limits.h includes

Closes https://github.com/curl/curl/pull/9453
diff --git a/lib/timediff.c b/lib/timediff.c
index 27fd911..c589318 100644
--- a/lib/timediff.c
+++ b/lib/timediff.c
@@ -24,6 +24,8 @@
 
 #include "timediff.h"
 
+#include <limits.h>
+
 /*
  * Converts number of milliseconds into a timeval structure.
  *
diff --git a/lib/warnless.c b/lib/warnless.c
index 51187aa..b00d7a5 100644
--- a/lib/warnless.c
+++ b/lib/warnless.c
@@ -39,6 +39,8 @@
 
 #include "warnless.h"
 
+#include <limits.h>
+
 #define CURL_MASK_UCHAR   ((unsigned char)~0)
 #define CURL_MASK_SCHAR   (CURL_MASK_UCHAR >> 1)