Reduce visibility on secp-sys symbols

cc-rs builds C dependencies with reduced visibility to avoid
exporting the C symbols all the way out to any rust-built shared
libraries however we override it with SECP256K1_API. We should
avoid doing this, allowing LTO/DCE to do its work.

secp256k1-sys/depend/secp256k1/include/secp256k1.h

@@ -135,7 +135,7 @@ typedef int (*rustsecp256k1_v0_4_0_nonce_function)(
 #   define SECP256K1_API
 #  endif
 # elif defined(__GNUC__) && (__GNUC__ >= 4) && defined(SECP256K1_BUILD)
-#  define SECP256K1_API __attribute__ ((visibility ("default")))
+#  define SECP256K1_API
 # else
 #  define SECP256K1_API
 # endif