ffi: explicitly declare hello_from_rust for C99 (#343)

Author: mqy

src/ffi.md

@@ -281,6 +281,8 @@ We'll create a C file to call the `hello_from_rust` function and compile it by `
 C file should look like:
 
 ```c
+extern void hello_from_rust();
+
 int main(void) {
     hello_from_rust();
     return 0;