Include buffer.h from bio.h.

We shouldn't really have to do this, but there's a lot of code that
doesn't always include what it uses. In this case, since bio.h
references |BUF_MEM| in function signatures, it seems a little less
distasteful.

Change-Id: Ifb50f8bce40639f977b4447404597168a68c8388
Reviewed-on: https://boringssl-review.googlesource.com/7361
Reviewed-by: David Benjamin <davidben@google.com>

include/openssl/bio.h

diff --git a/include/openssl/bio.h b/include/openssl/bio.h
index 75afada..2168812 100644
--- a/include/openssl/bio.h
+++ b/include/openssl/bio.h
@@ -61,6 +61,7 @@
 
 #include <stdio.h>  /* For FILE */
 
+#include <openssl/buffer.h>
 #include <openssl/err.h> /* for ERR_print_errors_fp */
 #include <openssl/ex_data.h>
 #include <openssl/stack.h>