encode_ratio_ism_subframe(): Compiler warning with DEBUGGING disabled

With DEBUGGING disabled, there is a compiler warning in encode_ratio_ism_subframe():

lib_enc/ivas_masa_enc.c: In function 'encode_ratio_ism_subframe':
lib_enc/ivas_masa_enc.c:3258:59: warning: variable 'bits_pos0' set but not used [-Wunused-but-set-variable]
 3258 |     int16_t nbits, nbits0, nbits1, GR_order, GR_order_sb, bits_pos0;
      |                                                           ^~~~~~~~~