sim: warnings: disable -Wenum-conversion fow now [PR sim/29752]

sim: warnings: disable -Wenum-conversion fow now [PR sim/29752]

The cgen code mixes virtual insn enums with insn enums, and there isn't
an obvious (to me) way to unravel this atm, so disable the warning.

sim/lm32/decode.c:45:5: error:
	implicit conversion from enumeration type 'CGEN_INSN_VIRTUAL_TYPE'
	to different enumeration type 'CGEN_INSN_TYPE' (aka 'enum cgen_insn_type')
	[-Werror,-Wenum-conversion]
   45 |   { VIRTUAL_INSN_X_INVALID, LM32BF_INSN_X_INVALID, LM32BF_SFMT_EMPTY },
      |   ~ ^~~~~~~~~~~~~~~~~~~~~~

Bug: https://sourceware.org/PR29752