intel/gen_decoder: Fix unused-but-set-variable warning

intel/gen_decoder: Fix unused-but-set-variable warning

This commit fixes the following warning:
../src/intel/common/gen_decoder.c: In function ‘gen_spec_load_from_path’:
../src/intel/common/gen_decoder.c:741:11: warning: variable ‘len’ set but not used [-Wunused-but-set-variable]
  741 |    size_t len, filename_len = strlen(path) + 20;
      |           ^~~

Signed-off-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>