fix auto-generated GDN API header for C compilers (#12891)

* fix auto-generated GDN API header for C compilers

modules/gdnative/SCsub

@@ -40,11 +40,13 @@ def _build_gdnative_api_struct_header(api):
         '\tunsigned int minor;',
         '} godot_gdnative_api_version;',
         '',
-        'typedef struct godot_gdnative_api_struct {',
+        'typedef struct godot_gdnative_api_struct godot_gdnative_api_struct;',
+        '',
+        'struct godot_gdnative_api_struct {',
         '\tunsigned int type;',
         '\tgodot_gdnative_api_version version;',
         '\tconst godot_gdnative_api_struct *next;',
-        '} godot_gdnative_api_struct;',
+        '};',
         '',
         'enum GDNATIVE_API_TYPES {',
         '\tGDNATIVE_' + api['core']['type'] + ','