build: fix ci failure due to partial missing bg in windows installer

Bulgarian was added to the installer po files and LINGUAS, but the other
files needing updates were forgotten. Let's fix that.

build/windows/installer/gimp3264.iss

@@ -187,6 +187,7 @@ PrivilegesRequiredOverridesAllowed=dialog
 
 [Languages]
 Name: "en"; MessagesFile: "compiler:Default.isl,lang\en.setup.isl"
+Name: "bg"; MessagesFile: "compiler:Languages\Bulgarian.isl,lang\bg.setup.isl"
 Name: "ca"; MessagesFile: "compiler:Languages\Catalan.isl,lang\ca.setup.isl"
 Name: "cs"; MessagesFile: "compiler:Languages\Czech.isl,lang\cs.setup.isl"
 Name: "da"; MessagesFile: "compiler:Languages\Danish.isl,lang\da.setup.isl"

build/windows/installer/lang/Makefile.am

@@ -13,6 +13,7 @@ $(isl_xml_files): $(isl_xml_in_files) $(wildcard $(top_srcdir)/po-windows-instal
 
 # Then, we generate the .isl file for each language using some xsltproc magic
 languages = \
+	bg:bg		\
 	ca:ca		\
 	cs:cs		\
 	da:da		\

build/windows/installer/lang/meson.build

@@ -8,6 +8,7 @@ ms_installer_config_xml = i18n.merge_file(
 )
 
 languages = [
+  { 'code': 'bg',    },
   { 'code': 'ca',    },
   { 'code': 'cs',    },
   { 'code': 'da',    },