Merge #818: secp256k1-sys: Add memmove to the wasm-sysroot header

c74c086 secp256k1-sys: Add memmove to the wasm-sysroot header (Tobin C. Harding)

Pull request description:

  WASM build is currently failing because we don't include `memmove` in our custom WASM header.
  
  Close: #817


ACKs for top commit:
  apoelstra:
    ACK c74c086; successfully ran local tests


Tree-SHA512: 24249fc61337ae77c0d2928dc57bb72aebb6f0876b29fe5b2e865e06586e2c2841c921ecbdb13a35eae264b1575fe9140a0e2d667d00a6af4703450c952a9e93

Author: apoelstra

secp256k1-sys/wasm/wasm-sysroot/string.h

@@ -2,3 +2,4 @@
 void *memset(void *s, int c, size_t n);
 void *memcpy(void *dest, const void *src, size_t n);
 int memcmp(const void *s1, const void *s2, size_t n);
+void *memmove(void *dest_str, const void *src_str, size_t numBytes);