Merge pull request #289 from TheBlueMatt/2021-03-no-vis

Reduce visibility on secp-sys symbols

secp256k1-sys/build.rs

@@ -33,6 +33,7 @@ fn main() {
                .include("depend/secp256k1/src")
                .flag_if_supported("-Wno-unused-function") // some ecmult stuff is defined but not used upstream
                .define("SECP256K1_BUILD", Some("1"))
+               .define("SECP256K1_API", Some(""))
                .define("ENABLE_MODULE_ECDH", Some("1"))
                .define("ENABLE_MODULE_SCHNORRSIG", Some("1"))
                .define("ENABLE_MODULE_EXTRAKEYS", Some("1"))