external/boringssl: export EC_GROUP_set_point_conversion_form symbol.

In https://android-review.googlesource.com/#/c/147551/, I missed the
OPENSSL_EXPORT tag thus the .so doesn't expose it as a dynamic symbol.

BUG=20419899

Change-Id: I849888cf9a3383570b352911867e983b547e6742
diff --git a/src/include/openssl/ec.h b/src/include/openssl/ec.h
index 36a3a29..2662c01 100644
--- a/src/include/openssl/ec.h
+++ b/src/include/openssl/ec.h
@@ -305,8 +305,8 @@
 
 /* EC_GROUP_set_point_conversion_form aborts the process if |form| is not
  * |POINT_CONVERSION_UNCOMPRESSED| and otherwise does nothing. */
-void EC_GROUP_set_point_conversion_form(EC_GROUP *group,
-                                        point_conversion_form_t form);
+OPENSSL_EXPORT void EC_GROUP_set_point_conversion_form(
+    EC_GROUP *group, point_conversion_form_t form);
 
 
 /* Old code expects to get EC_KEY from ec.h. */