meson: fix race condition revealed by using 0.44

meson: fix race condition revealed by using 0.44

Previously there was a special target that blocked for the generation of
anv_entrypoints.h, with meson 0.44 we don't need this, we can use a new
language feature instead. The problem is that previously that blocking
target would hide a race condition for the generation of another header,
anv_extensions.h. Now the build sometimes fails when anv_extensions.h is
not generated in time.

v2: - clarify the race condition in the commit message (Emil)

CC: Mark Janes <mark.a.janes@intel.com>
Fixes: 92550d9b16d2b295bdac087f31b1fd6d0f808e02
       ("meson: remove workaround for custom target creating .h and .c files")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Scott D Phillips <scott.d.phillips@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>