glsl: add fallthrough comment on switch

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-By: Martin Peres <martin.peres@linux.intel.com>
diff --git a/src/glsl/linker.cpp b/src/glsl/linker.cpp
index b6baa5d..9aa1389 100644
--- a/src/glsl/linker.cpp
+++ b/src/glsl/linker.cpp
@@ -2570,7 +2570,8 @@
          if (var->data.location != SYSTEM_VALUE_VERTEX_ID &&
              var->data.location != SYSTEM_VALUE_VERTEX_ID_ZERO_BASE &&
              var->data.location != SYSTEM_VALUE_INSTANCE_ID)
-         continue;
+            continue;
+            /* FALLTHROUGH */
       case ir_var_shader_in:
          if (programInterface != GL_PROGRAM_INPUT)
             continue;