make sure #endif comment matches the macro name

src/stdlib/SDL_string.c

@@ -718,7 +718,7 @@ int SDL_memcmp(const void *s1, const void *s2, size_t len)
         ++s2p;
     }
     return 0;
-#endif // HAVE_MEMCMP
+#endif // SDL_PLATFORM_VITA
 }
 
 size_t SDL_strlen(const char *string)
@@ -1119,7 +1119,7 @@ char *SDL_strnstr(const char *haystack, const char *needle, size_t maxlen)
         --maxlen;
     }
     return NULL;
-#endif // HAVE_STRSTR
+#endif // HAVE_STRNSTR
 }
 
 char *SDL_strstr(const char *haystack, const char *needle)