Scanner: add type specifier missing

[aspeddro]

I got the following error when compile to wasm on CI:

src/scanner.c:154:9: error: type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int [-Wimplicit-int]
  const in_string = state->in_quotes || state->in_backticks;
  ~~~~~ ^
  int
1 error generated.
emcc: error: '/emsdk/upstream/bin/clang -target wasm32-unknown-emscripten -fignore-exceptions -fPIC -fvisibility=default -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -DEMSCRIPTEN -Werror=implicit-function-declaration -I/emsdk/upstream/emscripten/cache/sysroot/include/SDL --sysroot=/emsdk/upstream/emscripten/cache/sysroot -Xclang -iwithsysroot/include/compat -Os -fno-exceptions -Isrc src/scanner.c -c -o /tmp/emscripten_temp_kz9rxsh8/scanner_0.o' failed (returned 1)

[nkrkv]

Thx!