glsl/meson: Add variable to export float64.glsl

Signed-off-by: Mykhailo Skorokhodov <mykhailo.skorokhodov@globallogic.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18854>

src/compiler/glsl/meson.build

@@ -70,6 +70,8 @@ float64_glsl_h = custom_target(
   command : [prog_python, '@INPUT@', '@OUTPUT@', '-n', 'float64_source'],
 )
 
+float64_glsl_file = [files('float64.glsl')]
+
 files_libglsl = files(
   'ast.h',
   'ast_array_index.cpp',