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
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32610

Author: simark

gdb/darwin-nat.c

@@ -26,6 +26,7 @@
 #include "symtab.h"
 #include "objfiles.h"
 #include "cli/cli-cmds.h"
+#include "cli/cli-style.h"
 #include "gdbcore.h"
 #include "gdbthread.h"
 #include "regcache.h"