intel/meson: Fix warning about broken str.format

Meson complains:

../src/intel/decoder/meson.build:67: DEPRECATION: Project uses feature that was always broken, and is now deprecated since '1.3.0': str.format: Value other than strings, integers, bools, options, dictionaries and lists thereof..

So instead of trying to format a file, change gentest_xml to store just
the string.  Need to adapt genxml_path to consider the current source
dir, but everything else works like before.

Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27706>

src/intel/decoder/meson.build

@@ -36,7 +36,7 @@ idep_intel_decoder = declare_dependency(
 )
 
 if with_tests and not with_platform_android
-  gentest_xml = files('tests/gentest.xml')
+  gentest_xml = 'tests/gentest.xml'
   _name = 'gentest_pack.h'
   gentest_pack = custom_target(
     _name,
@@ -47,8 +47,8 @@ if with_tests and not with_platform_android
     depend_files: gen_pack_header_deps
   )
 
-  genxml_path = join_paths(dir_source_root,
-                           '@0@'.format(gentest_xml[0]))
+  genxml_path = join_paths(meson.current_source_dir(),
+                           '@0@'.format(gentest_xml))
 
   test(
     'genxml_test',