meson: fix HAVE_LLVM version define in meson build

LLVM patch level is not included in HAVE_LLVM.

Fixes: e6418ab1566d ("meson: build "radv" vulkan driver for radeon hardware")
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Dylan Baker <dylan.c.baker@intel.com>
Signed-off-by: Marc Dietrich <marvin24@gmx.de>
(cherry picked from commit a2a1b0e7)