error when run g++ src/pngc.cpp -o pngc

system: WSL on win11

```shell
linq@linq-desktop:/mnt/d/pros/png-optimizer$ g++ src/pngc.cpp -o pngc
src/pngc.cpp: In function ‘bool compressPNG(char*)’:
src/pngc.cpp:64:72: error: narrowing conversion of ‘137’ from ‘int’ to ‘char’ inside { } [-Wnarrowing]
         const char fileSignature[] = { 137, 80, 78, 71, 13, 10, 26, 10 };
```