Use flag when building secp sys

We currently use the `cc::Build::flag_if_supported` function to pass in
`-Wno-unused-function` however when building for wasm the flag is
ignored.

Using plain old `flag` seems to work, found by trial and error, I did no
further investigation as to why it works and the other does not.

Resolves: rust-bitcoin#434

secp256k1-sys/build.rs

@@ -19,7 +19,7 @@ fn main() {
     base_config.include("depend/secp256k1/")
                .include("depend/secp256k1/include")
                .include("depend/secp256k1/src")
-               .flag_if_supported("-Wno-unused-function") // some ecmult stuff is defined but not used upstream
+               .flag("-Wno-unused-function") // some ecmult stuff is defined but not used upstream
                .define("SECP256K1_API", Some(""))
                .define("ENABLE_MODULE_ECDH", Some("1"))
                .define("ENABLE_MODULE_SCHNORRSIG", Some("1"))