Adding mising <string> header.

Fixing error:

fuzz/ssl_ctx_api.cc:232:6: error: implicit instantiation of undefined
template 'std::__1::basic_string....

BUG=

Change-Id: I6d623dcca3e4edc52702d713fc948a0242bd4db8
Reviewed-on: https://boringssl-review.googlesource.com/11540
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/fuzz/ssl_ctx_api.cc b/fuzz/ssl_ctx_api.cc
index 736cf54..44078f8 100644
--- a/fuzz/ssl_ctx_api.cc
+++ b/fuzz/ssl_ctx_api.cc
@@ -13,6 +13,7 @@
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
 
 #include <functional>
+#include <string>
 #include <vector>
 
 #include <assert.h>