tests: suppress -Wmissing-variable-declarations in viewer test stubs

Add extern forward declarations for global linker stub variables
(mc_skin_color__cache, mc_editor_plugin_list, we_are_strstrstrbackground)
to satisfy -Wmissing-variable-declarations. These variables must remain
non-static as they are referenced by name from libinternal.a objects.

Signed-off-by: Ilia Maslakov <il.smind@gmail.com>

Author: ilia-maslakov

tests/src/viewer/mcview__load_zip_magic.c

@@ -41,13 +41,19 @@
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wmissing-prototypes"
 
+extern int mc_skin_color__cache[64];
 int mc_skin_color__cache[64];
-void tty_lowlevel_setcolor (MC_UNUSED int color) {}
+
+extern void *mc_editor_plugin_list;
 void *mc_editor_plugin_list = NULL;
+
+extern int we_are_strstrstrbackground;
+int we_are_strstrstrbackground = 0;
+
+void tty_lowlevel_setcolor (MC_UNUSED int color) {}
 void mc_editor_plugin_add (MC_UNUSED void *p) {}
 void mc_editor_plugins_load (void) {}
 int button_get_width (MC_UNUSED void *b) { return 0; }
-int we_are_strstrstrbackground = 0;
 
 #pragma GCC diagnostic pop