Fixed missing reference to strchrnul on macOS

```
logging.c:651:28: error: 'strchrnul' is only available on macOS 15.4 or newer [-Werror,-Wunguarded-availability-new]
  651 |         char *next_token = strchrnul(token, ',');
      |                            ^~~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/_string.h:198:9: note: 'strchrnul' has been marked as being introduced in macOS 15.4 here, but the deployment target is macOS 15.0.0
  198 |         strchrnul(const char *__s, int __c);
      |         ^
logging.c:651:28: note: enclose 'strchrnul' in a __builtin_available check to silence this warning
  651 |         char *next_token = strchrnul(token, ',');
      |                            ^~~~~~~~~
1 error generated.
```

Signed-off-by: Lars Erik Wik <lars.erik.wik@northern.tech>

Author: larsewi

libutils/platform.h

@@ -229,6 +229,19 @@ ULONGLONG WINAPI GetTickCount64(void);
 
 #endif
 
+#if !HAVE_STRCHRNUL
+/**
+ * @brief The strchrnul() function is like strchr() except that if c is not
+ * found in s, then it returns a pointer to the null byte at the end of s,
+ * rather than NULL.
+ */
+static inline char *strchrnul(const char *s, int c)
+{
+    const char *r = strchr(s, c);
+    return (r == NULL) ? (s + (strlen(s) - 1)) : r;
+}
+#endif
+
 #if !HAVE_DECL_DIRFD
 int dirfd(DIR *dirp);
 #endif