style: Properly handle exhaustive matches in animation keywords to avoid a warning.

Need to do the gecko_inexhaustive thing because those animation properties have
an EndGuard_ generated by IPDL I suspect.

components/style/properties/gecko.mako.rs

@@ -3000,7 +3000,9 @@ fn static_assert() {
             % for value in keyword.gecko_values():
                 structs::${keyword.gecko_constant(value)} => Keyword::${to_camel_case(value)},
             % endfor
+            % if keyword.gecko_inexhaustive:
             _ => panic!("Found unexpected value for animation-${ident}"),
+            % endif
         }
     }
     ${impl_animation_count(ident, gecko_ffi_name)}

components/style/properties/longhands/box.mako.rs

@@ -247,6 +247,7 @@ ${helpers.single_keyword(
     gecko_enum_prefix="PlaybackDirection",
     custom_consts=animation_direction_custom_consts,
     extra_prefixes=animation_extra_prefixes,
+    gecko_inexhaustive=True,
     spec="https://drafts.csswg.org/css-animations/#propdef-animation-direction",
     allowed_in_keyframe_block=False,
 )}
@@ -271,6 +272,7 @@ ${helpers.single_keyword(
     vector=True,
     gecko_enum_prefix="FillMode",
     extra_prefixes=animation_extra_prefixes,
+    gecko_inexhaustive=True,
     spec="https://drafts.csswg.org/css-animations/#propdef-animation-fill-mode",
     allowed_in_keyframe_block=False,
 )}