Fix libpng LoongArch LSX undefined symbol in native builds

[mattleibow]

Problem

When building libSkiaSharp for loongarch64 with -Wl,--no-undefined (from #3629), the linker fails:

undefined reference to `png_init_filter_functions_lsx'

This is a pre-existing latent bug — the loongarch64 cross-compiler defines __loongarch_sx, causing libpng's pngpriv.h to enable PNG_LOONGARCH_LSX_OPT=1 and reference png_init_filter_functions_lsx(). However, the implementation files were never included in Skia's GN build. Previously this was silently tolerated by the linker; the --no-undefined flag correctly surfaces it.

Fix

Updates the skia submodule to include mono/skia#176, which adds the missing LoongArch LSX source files (filter_lsx_intrinsics.c, loongarch_lsx_init.c) to the libpng GN build for current_cpu == "loongarch64".

Related

• Enforce --no-undefined linker flag for Linux and Android native builds #3629 — --no-undefined linker flag PR that exposed this issue
• Enforce --no-undefined linker flag and fix libpng LoongArch LSX build skia#176 — the actual source fix in the skia submodule

[mattleibow]

Closing — folding the submodule update into #3629 instead.