Remove string.h from base.h.

Including string.h in base.h causes any file that includes a BoringSSL
header to include string.h. Generally this wouldn't be a problem,
although string.h might slow down the compile if it wasn't otherwise
needed. However, it also causes problems for ipsec-tools in Android
because OpenSSL didn't have this behaviour.

This change removes string.h from base.h and, instead, adds it to each
.c file that requires it.

Change-Id: I5968e50b0e230fd3adf9b72dd2836e6f52d6fb37
Reviewed-on: https://boringssl-review.googlesource.com/3200
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

crypto/bio/bio_mem.c

diff --git a/crypto/bio/bio_mem.c b/crypto/bio/bio_mem.c
index 6e90db5..f3aad6f 100644
--- a/crypto/bio/bio_mem.c
+++ b/crypto/bio/bio_mem.c
@@ -57,6 +57,7 @@
 #include <openssl/bio.h>
 
 #include <limits.h>
+#include <string.h>
 
 #include <openssl/buf.h>
 #include <openssl/err.h>