Add ifdef guards to wasm-sysroot headers

In typical C fashion headers should have if guards to stop them being
included multiple times in an object file.

Done in an attempt to fix the current WASM build failures e.g.,

 rust-lld: error: duplicate symbol: WASM32_INT_SIZE
 >>> defined in /home/tobin/.cache/cargo/wasm32-unknown-unknown/release/deps/libsecp256k1_sys-26edc0990a3ac6e5.rlib(precomputed_ecmult_gen.o)
 >>> defined in /home/tobin/.cache/cargo/wasm32-unknown-unknown/release/deps/libsecp256k1_sys-26edc0990a3ac6e5.rlib(precomputed_ecmult.o)

This seems valuable to have even if it does not fix the build issue.

secp256k1-sys/wasm-sysroot/stdio.h

@@ -1,3 +1,6 @@
+#ifndef WASM_SYSROOT_STDIO_H
+#define WASM_SYSROOT_STDIO_H
+
 #include <stddef.h>
 #define alignof(type) offsetof (struct { char c; type member; }, member)
 
@@ -14,4 +17,6 @@ extern const unsigned char WASM32_UNSIGNED_CHAR_SIZE = sizeof(unsigned char);
 extern const unsigned char WASM32_UNSIGNED_CHAR_ALIGN = alignof(unsigned char);
 
 extern const unsigned char WASM32_PTR_SIZE = sizeof(void*);
-extern const unsigned char WASM32_PTR_ALIGN = alignof(void*);
+extern const unsigned char WASM32_PTR_ALIGN = alignof(void*);
+
+#endif  /* WASM_SYSROOT_STDIO_H */

secp256k1-sys/wasm-sysroot/string.h

@@ -1,4 +1,9 @@
+#ifndef WASM_SYSROOT_STRING_H
+#define WASM_SYSROOT_STRING_H
+
 #include <stddef.h>
 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);
+
+#endif  /* WASM_SYSROOT_STRING_H */