build: Check for PyYAML in Meson build

Closes: #11540
Fixes: ccc6442d6f ("u_format: Rewrite format table to use YAML")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30272>
(cherry picked from commit bed6e0d691)

.pick_status.json

@@ -264,7 +264,7 @@
         "description": "build: Check for PyYAML in Meson build",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "ccc6442d6f0539571c413222e0d6aadc78bc5d4d",
         "notes": null

meson.build

@@ -952,6 +952,15 @@ if has_mako.returncode() != 0
   error('Python (3.x) mako module >= 0.8.0 required to build mesa.')
 endif
 
+has_yaml = run_command(
+  prog_python, '-c',
+  '''
+import yaml
+  ''', check: false)
+if has_yaml.returncode() != 0
+  error('Python (3.x) yaml module (PyYAML) required to build mesa.')
+endif
+
 if cc.get_id() == 'gcc' and cc.version().version_compare('< 4.4.6')
   error('When using GCC, version 4.4.6 or later is required.')
 endif