Compile C API (src/c) in libSkiaSharp on all platforms

Follow-up to the mono/skia change that moves the fork-owned C API shim
(src/c/*.cpp) out of upstream Skia's :core target and into the libSkiaSharp
build, decoupling it from :core's milestone-churning transitive dependencies
(which silently dropped SK_FONTMGR_FONTCONFIG_AVAILABLE on Linux in m148).

On platforms built via the GN skiasharp_build("SkiaSharp") target (Android,
Linux, Windows, WASM) the move is handled entirely in the submodule. But the
Apple platforms (macOS, iOS, tvOS) and Tizen build libSkiaSharp from a project
file that compiles the managed-interop glue and links the GN-built libskia.a;
they relied on src/c being inside :core. Add the src/c/*.cpp sources to:

  - native/{macos,ios,tvos}/libSkiaSharp/libSkiaSharp.xcodeproj
  - native/tizen/libSkiaSharp/project_def.prop

so every platform compiles the C API into libSkiaSharp itself, consistent with
how the src/xamarin glue is already handled. These projects already link
-lskottie/-lsksg/-lskresources/-ljsonreader, so no extra linkage is needed.

Validated: macOS arm64 (real dylib, full test suite passes) and Linux x64
(real .so with libfontconfig.so.1 dependency and 44 Fc* imports — the m148
regression guard).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mattleibow