Fix `CAPNP_LITE` build

Building with `--disable-reflection` fails when compiling `schema.capnp.c++`:

```
libtool: compile:  g++ -std=gnu++11 -DHAVE_CONFIG_H -I. -I./src -I./src -DCAPNP_INCLUDE_DIR=\"/usr/local/include\" -D_THREAD_SAFE -DCAPNP_LITE -O2 -DNDEBUG -D_THREAD_SAFE -MT src/capnp/schema.capnp.lo -MD -MP -MF src/capnp/.deps/schema.capnp.Tpo -c src/capnp/schema.capnp.c++  -fno-common -DPIC -o src/capnp/.libs/schema.capnp.o
In file included from src/capnp/schema.capnp.c++:4:
src/capnp/schema.capnp.h:90:46: error: too few arguments provided to function-like macro invocation
  CAPNP_DECLARE_STRUCT(e682ab4cf923a417, 5, 6);
                                             ^
./src/capnp/generated-header-support.h:484:9: note: macro 'CAPNP_DECLARE_STRUCT' defined here
#define CAPNP_DECLARE_STRUCT(id, dataWordSize, pointerCount, preferredElementEncoding) \
        ^
In file included from src/capnp/schema.capnp.c++:4:
src/capnp/schema.capnp.h:90:3: error: C++ requires a type specifier for all declarations
  CAPNP_DECLARE_STRUCT(e682ab4cf923a417, 5, 6);
  ^~~~~~~~~~~~~~~~~~~~
[...]
```

It looks like the changes in [b469079] weren't applied to both sides of the `#if CAPNP_LITE`
conditional.