fix: replace POSTJECT_SENTINEL_STRING macro with a const char*

Maybe that will fix this error:
```
Error: Could not find the sentinel POSTJECT_SENTINEL_fce680ab2cc467b6e072b8b5df1996b2: in the binary
    at inject (dist/api.js:5147:11)
    at async Context.<anonymous> (file:///home/circleci/project/test/cli.mjs:138:7)
```

Refs: https://app.circleci.com/pipelines/github/postmanlabs/postject/197/workflows/741145b6-e19e-4d09-b995-49eebc237962/jobs/1580
Signed-off-by: Darshan Sen <raisinten@gmail.com>

postject-api.h

@@ -37,10 +37,8 @@ inline void postject_options_init(struct postject_options* options) {
 }
 
 static inline bool postject_has_resource() {
-#define POSTJECT_SENTINEL_STRING \
-  "POSTJECT_SENTINEL_fce680ab2cc467b6e072b8b5df1996b2:0"
-  return POSTJECT_SENTINEL_STRING[sizeof(POSTJECT_SENTINEL_STRING) - 2] == '1';
-#undef POSTJECT_SENTINEL_STRING
+  const char* sentinel = "POSTJECT_SENTINEL_fce680ab2cc467b6e072b8b5df1996b2:0";
+  return sentinel[strlen(sentinel) - 1] == '1';
 }
 
 static const void* postject_find_resource(