ffi: explicitly declare hello_from_rust for C99

Author: mqy

src/ffi.md

@@ -281,6 +281,10 @@ We'll create a C file to call the `hello_from_rust` function and compile it by `
 C file should look like:
 
 ```c
+// Explicitly declare to avoid error like this:
+// implicit declaration of function 'hello_from_rust' is invalid in C99 ...
+extern void hello_from_rust();
+
 int main(void) {
     hello_from_rust();
     return 0;