Fix error when building with Clang

[wezm]

Building scanner.c with clang fails with this error:

src/scanner.c:134:9: 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.

This PR fixes the error

[jaredramirez]

Thanks for this!