devel-docs: fix failure to build gir docs

Generating the devel docs fails on Windows because it can't find
Babl-0.1.gir, which is only available for me in GIMP's prefix.

Adding that path with -add-include-path fixes the issue for me.

devel-docs/reference/gimp-ui/meson.build

@@ -25,6 +25,7 @@ gimp_ui_docs = custom_target('gimp-ui-docs',
     '--no-namespace-dir',
     '--content-dir=@0@'.format(meson.current_source_dir()),
     '--add-include-path=@0@'.format(meson.project_build_root() / 'libgimp'),
+    '--add-include-path=@0@'.format(get_option('prefix') / 'share' / 'gir-1.0'),
     '@INPUT@',
   ],
   depends: libgimp_gir[0],

devel-docs/reference/gimp/meson.build

@@ -23,6 +23,7 @@ gimp_docs = custom_target('gimp-docs',
     '--output-dir=@OUTPUT@',
     '--no-namespace-dir',
     '--content-dir=@0@'.format(meson.current_source_dir()),
+    '--add-include-path=@0@'.format(get_option('prefix') / 'share' / 'gir-1.0'),
     '@INPUT@',
   ],
   depend_files: [