Import custom WASM header

`sanit_checks_for_wasm()` is failing because the WASM consts are not
defined. They are defined in a custom header, we can now include this in
the vendored `libsecp256k1` code so that our unit tests pass.

secp256k1-sys/depend/secp256k1/src/secp256k1.c

@@ -8,6 +8,7 @@
 
 #include "../include/secp256k1.h"
 #include "../include/secp256k1_preallocated.h"
+#include "../../../wasm-sysroot/wasm.h"
 
 #include "assumptions.h"
 #include "util.h"