gdb: remove local extern declaration of cli_styling

gdb: remove local extern declaration of cli_styling

Following the int -> bool conversion of boolean options (commit
491144b5e21b ("Change boolean options to bool instead of int")), I see
this ASAN error:

  ==357543==ERROR: AddressSanitizer: global-buffer-overflow on address 0x55555b25d440 at pc 0x5555583ce9e1 bp 0x7fffffffd390 sp 0x7fffffffd380
  READ of size 4 at 0x55555b25d440 thread T0
      #0 0x5555583ce9e0 in term_cli_styling /home/simark/src/binutils-gdb/gdb/ui-file.c:111
      #1 0x5555583cf8b0 in stdio_file::can_emit_style_escape() /home/simark/src/binutils-gdb/gdb/ui-file.c:308
      #2 0x5555584450d2 in set_output_style /home/simark/src/binutils-gdb/gdb/utils.c:1442
      #3 0x5555584491af in fprintf_styled(ui_file*, ui_file_style const&, char const*, ...) /home/simark/src/binutils-gdb/gdb/utils.c:2143
      #4 0x5555582fa13c in print_gdb_version(ui_file*, bool) /home/simark/src/binutils-gdb/gdb/top.c:1339
      #5 0x555557b723ab in captured_main_1 /home/simark/src/binutils-gdb/gdb/main.c:981
      #6 0x555557b7353b in captured_main /home/simark/src/binutils-gdb/gdb/main.c:1172
      #7 0x555557b735d0 in gdb_main(captured_main_args*) /home/simark/src/binutils-gdb/gdb/main.c:1197
      #8 0x55555700a53d in main /home/simark/src/binutils-gdb/gdb/gdb.c:32
      #9 0x7ffff64c9ee2 in __libc_start_main (/usr/lib/libc.so.6+0x26ee2)
      #10 0x55555700a30d in _start (/home/simark/build/binutils-gdb/gdb/gdb+0x1ab630d)

  0x55555b25d441 is located 0 bytes to the right of global variable 'cli_styling' defined in '/home/simark/src/binutils-gdb/gdb/cli/cli-style.c:31:6' (0x55555b25d440) of size 1

The reason of this is that we have this declaration of cli_styling in cli/cli-style.h:

  extern bool cli_styling;

but ui-file.c uses its  own local declaration:

  extern int cli_styling;

Because of that, the code in ui-file.c thinks the variable is 4 bytes
long, when it is actually 1 byte long, so the generated code reads past
the variable.

Fix it by removing the declaration and making ui-file.c include
cli/cli-style.h.

gdb/ChangeLog:

	* ui-file.c: Include cli/cli-style.h.
	(term_cli_styling): Remove cli_styling declaration.