util/StringAPI: add strncmp() wrapper

src/util/StringAPI.hxx

 /*
- * Copyright 2010-2018 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2010-2019 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -134,6 +134,13 @@ StringCompare(const char *a, const char *b) noexcept
 	return strcmp(a, b);
 }
 
+gcc_pure gcc_nonnull_all
+static inline int
+StringCompare(const char *a, const char *b, size_t n) noexcept
+{
+	return strncmp(a, b, n);
+}
+
 /**
  * Checks whether #a and #b are equal.
  */