openocd: fix build with 'configure --without-capstone'

openocd: fix build with 'configure --without-capstone'

When configure option --without-capstone is used, the macro
HAVE_CAPSTONE is not defined in config.h, and the following lines
are instead present:
	/* 1 if you have Capstone disassembly framework. */
	/* #undef HAVE_CAPSTONE */

This cause compile error with message:
	arm_disassembler.h:190:5: error: "HAVE_CAPSTONE" is not
		defined, evaluates to 0 [-Werror=undef]
	  190 | #if HAVE_CAPSTONE
	      |     ^~~~~~~~~~~~~

This is caused by configure.ac that does not call AC_DEFINE when
--without-capstone option is present.

Fix configure.ac to always provide the autoconf macro
HAVE_CAPSTONE, with either value 0 or 1.

Change-Id: Ie5ac98b2c25746dd721812c91baaac61ec877ecd
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7224
Tested-by: jenkins