gdb: include cli/cli-style.h in darwin-nat.c

gdb: include cli/cli-style.h in darwin-nat.c

PR 32610 says:

  File gdb/darwin-nat.c is missing an #include statement of
  "cli/cli-style.h". It is needed because there is a reference to class
  object command_style in the .c file.

I'm not able to build-test this change (I only have access to arm64
macos machines, which GDB doesn't support yet), but I don't think I'm
doing things worse by adding this.

Change-Id: I2a169664ff91b92caf27cb084334f2eb4df46aa5