tracing: fix MSVC LNK2019 in DataSourceHelper::type() (#5591)

Move the primary-template definition of `DataSourceHelper::type()` out of
the class body so it is not implicitly inline.

`DataSourceHelper::type()` was added in 06486b8b6010 ("Fix component build
for client library on Windows", 2023-05-05) as a workaround for an MSVC
C2086 redefinition bug affecting an earlier static-data-member design.
That commit defined `type()` inline inside the class body
([dcl.fct.spec]/4 then makes it implicitly inline). MSVC propagates that
inline attribute to every explicit specialization of `type()` defined
out-of-line by `PERFETTO_DEFINE_DATA_SOURCE_STATIC_MEMBERS` in a `.cc`,
emits a C4506 ("no definition for inline function"), and silently drops
the specialization body. Any TU that does not see the body but calls
`Helper::type()` -- transitively via `DataSource<...>::Register<>()`,
`Trace()`, etc. -- is then left with an unresolved external symbol.

The bug stayed dormant until a5f062d78a60 ("[API] Single data source for
all track events", 2025-06-24, in v52.0+) made `TrackEventDataSource`
a concrete class with a `PERFETTO_DECLARE_DATA_SOURCE_STATIC_MEMBERS`
visible in the public header, so user TUs that invoke
`PERFETTO_TRACK_EVENT_STATIC_STORAGE()` started instantiating
`Register<>` and producing LNK2019:

  example.cc.obj : error LNK2019: unresolved external symbol
  "public: static class perfetto::internal::DataSourceType & __cdecl
   perfetto::DataSourceHelper<class perfetto::internal::TrackEventDataSource,
                              struct perfetto::internal::TrackEventDataSourceTraits>::type(void)"
  referenced in function "perfetto::DataSource<...>::Register<>(...)".

GCC and Clang are unaffected: they treat the out-of-line explicit
specialization as a regular non-inline function and emit it normally.

Fix: declare `type()` in the class body and define it out-of-line. The
primary template's member is then a non-inline function template, and
explicit specializations are unambiguously non-inline regular functions
on every compiler. No call sites change; the only file touched is
`include/perfetto/tracing/data_source.h`.

Verified on Windows / MSVC v143 (14.50.35717) for x64 Debug and Release
builds of `examples/sdk` and the v54.0 amalgamated SDK. GCC/Clang
unaffected because the change is a no-op semantic refactor for them.

Bug: #5591