samples/native-lib: Fix exec_env type (#1557)

Change `wasm_exec_env_t *exec_env` to `wasm_exec_env_t exec_env`

samples/native-lib/README.md

@@ -6,7 +6,7 @@ The native library should provide `get_native_lib` API for iwasm to return the n
 
 ```C
 static int
-foo_wrapper(wasm_exec_env_t *exec_env, int x, int y)
+foo_wrapper(wasm_exec_env_t exec_env, int x, int y)
 {
     return x + y;
 }

samples/native-lib/test_add.c

@@ -9,7 +9,7 @@
 #include "wasm_export.h"
 
 static int
-test_add_wrapper(wasm_exec_env_t *exec_env, int x, int y)
+test_add_wrapper(wasm_exec_env_t exec_env, int x, int y)
 {
     return x + y;
 }

samples/native-lib/test_sqrt.c

@@ -9,7 +9,7 @@
 #include "wasm_export.h"
 
 static int
-test_sqrt_wrapper(wasm_exec_env_t *exec_env, int x, int y)
+test_sqrt_wrapper(wasm_exec_env_t exec_env, int x, int y)
 {
     return x * x + y * y;
 }