Kernel: Add strncmp()

Author: https://github.com/bugaevc
Commit: 2396b2ed70
Pull-request: https://github.com/SerenityOS/serenity/pull/434
Reviewed-by: https://github.com/awesomekling

Kernel/StdLib.h

@@ -11,6 +11,7 @@ char* strcpy(char*, const char*);
 char* strncpy(char*, const char*, size_t);
 int strcmp(char const*, const char*);
 size_t strlen(const char*);
+size_t strnlen(const char*, size_t);
 void* memset(void*, int, size_t);
 char* strdup(const char*);
 int memcmp(const void*, const void*, size_t);