Add missing <errno.h> include.

Change-Id: I33d9dddbae277bcbfaeff6aacedd6d1371605207
Signed-off-by: Piotr Sikora <piotr@cloudflare.com>
Reviewed-on: https://boringssl-review.googlesource.com/1651
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/tool/pkcs12.cc b/tool/pkcs12.cc
index f6d7d10..10ff630 100644
--- a/tool/pkcs12.cc
+++ b/tool/pkcs12.cc
@@ -16,6 +16,7 @@
 #include <string>
 #include <vector>
 
+#include <errno.h>
 #include <fcntl.h>
 #include <stdint.h>
 #include <sys/stat.h>