[gdb/build] Fix x86_64-w64-mingw32 build by avoiding SCNx8

With an x86_64-w64-mingw32 targeted cross-build on x86_64-linux, I run into:
...
gdb/cli/cli-decode.c: \
  In function 'ui_file_style::color parse_cli_var_color(const char**)':
gdb/cli/cli-decode.c:2917:41: error: expected ')' before 'SCNx8'
  int parsed_args = sscanf (*args, "#%2" SCNx8 "%2" SCNx8 "%2" SCNx8 "%n",
...

Apparantly, the definition of SCNx8 is missing in inttypes.h.

Rewrite the sscanf call to use SCNx32, which is available.

Tested by:
- completing aforementioned cross-build, and
- build & test on x86_64-linux.

Suggested-By: Tom Tromey <tom@tromey.com>
Approved-By: Tom Tromey <tom@tromey.com>