bsp-compilation: build on Linux/GCC (missing includes, uint16_t, strdup)

[SickoHours]

What changed

Four small portability commits so bsp-compilation (head e4fb8b4) compiles on Linux with GCC 16 in the release_x86 configuration. No logic changes:

• src/ObjCommon/Shader/D3D11ShaderAnalyser.h: #include <cstdint> (upstream main already has this).
• src/ObjLoading/Game/T6/BSP/BSPUtil.cpp: remove a stray #pragma once from a .cpp (GCC errors under -Werror=pragma-once-outside-header).
• BSP/Linker/{BSPLinker,ClipMapLinker,GfxWorldLinker}.cpp: #include <cstring> for memset/memcpy; BSP/Linker/{ComWorldLinker,GfxWorldLinker}.cpp: #include <cmath> for cosf/roundf.
• BSP/Linker/GfxWorldLinker.cpp: unsigned __int16 (MSVC only) to uint16_t.
• src/ObjLoading/Game/T6/TechniqueSet/LoaderTechniqueSetT6.cpp: _strdup to strdup.

Evidence

Built with premake5 gmake && make -C build config=release_x86 Linker Unlinker (g++ 16.2.1, 32-bit multilib, premake 5.0 beta8). The resulting Linker (v0.24.1-296-ge4fb8b47) linked a minimal Zombies map from a Blender 5.2 GLB export (one closed room, sun, worldspawn, mp_global_intermission, four spawnpoint, one zone, four spawner nodes) into a zm_box.ff carrying gfxworld, clipmap, comworld, gameworldmp, mapents and skinnedverts; both the branch Unlinker and stock OAT 0.33 list it with 0 errors, and the dumped entity string has the expected structs.

Not verified

The map has not been loaded in the game yet, so this PR claims a build fix only. Two observations from the same session that may be useful, not addressed here: two identical links of the same inputs produce fastfiles differing in ~2200 bytes of the compressed body (headers equal), which suggests uninitialized memory somewhere in the BSP linkers; and a .gltf with an external .bin buffer is refused ("File buffers are not supported"), so GLB is the working export.

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