Fix SSL_COMP_get_compression_methods type signature.

We returned the wrong type, but with a typedef which made it void*. In
C++, void* to T* doesn't implicitly convert, so it doesn't quite work
right. Notably, Node passes it into sk_SSL_COMP_zero. The sk_* macros
only weakly typecheck right now, but a pending CL converts them to
proper functions.

Change-Id: I635d1e39e4f4f11b2b7bf350115a7f1b1be30e4f
Reviewed-on: https://boringssl-review.googlesource.com/16447
Reviewed-by: Adam Langley <agl@google.com>

ssl/ssl_cipher.c

diff --git a/ssl/ssl_cipher.c b/ssl/ssl_cipher.c
index da3f382..83dea45 100644
--- a/ssl/ssl_cipher.c
+++ b/ssl/ssl_cipher.c
@@ -1724,7 +1724,7 @@
   return "TLSv1/SSLv3";
 }
 
-COMP_METHOD *SSL_COMP_get_compression_methods(void) { return NULL; }
+STACK_OF(SSL_COMP) *SSL_COMP_get_compression_methods(void) { return NULL; }
 
 int SSL_COMP_add_compression_method(int id, COMP_METHOD *cm) { return 1; }