Issue #12717: file-bmp plugin broke cross-build with generated source

- build 'generate-huffman' natively when cross-building
- also reenable build of file-bmp for cross-builds

plug-ins/file-bmp/meson.build

@@ -1,7 +1,8 @@
 plugin_name = 'file-bmp'
 
 gen_huffman = executable('generate-huffman',
-                         'generate-huffman.c'
+                         'generate-huffman.c',
+                         native: true
 )
 
 huffman = custom_target('huffman.h',

plug-ins/meson.build

@@ -1,4 +1,5 @@
 subdir('common')
+subdir('file-bmp')
 subdir('file-dds')
 subdir('file-exr')
 subdir('file-faxg3')
@@ -31,7 +32,3 @@ subdir('screenshot')
 subdir('script-fu')
 subdir('selection-to-path')
 subdir('twain')
-
-if not meson.is_cross_build()
-  subdir('file-bmp')
-endif