sim: Fix compile errors

sim: Fix compile errors

The following change broke simulator testsuite with host GCC 13:
  commit 435ad222b3de93fa647fba7221eece36b1b395eb
  sim: warnings: compile build tools with -Werror too

Host GCC13 complains about missing function prototypes:

binutils/sim/testsuite/common/bits-gen.c:26:1: error: no previous prototype for ‘gen_struct’ [-Werror=missing-prototypes]
   26 | gen_struct (void)
      | ^~~~~~~~~~

Fix by making the functions static, which instructs the compiler that
there is no need for a prototype.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>