meson: bump required version to 0.46

0.45 has a few annoying bugs (like the one in !358 [1]), and 0.46 is
well over a year old by now, so let's move to it.

[1] https://gitlab.freedesktop.org/mesa/mesa/merge_requests/358

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

meson.build

@@ -25,7 +25,7 @@ project(
     [find_program('python', 'python2', 'python3'), 'bin/meson_get_version.py']
   ).stdout(),
   license : 'MIT',
-  meson_version : '>= 0.45',
+  meson_version : '>= 0.46',
   default_options : ['buildtype=debugoptimized', 'b_ndebug=if-release', 'c_std=c99', 'cpp_std=c++11']
 )