Add the includes clang-tidy's include cleaner asks for on the belt files

The clang-tidy job on #14394 fails on 129 misc-include-cleaner findings:
the belt sources and tests use std::, Eigen, Point/PrintConfig and
BeltBrim symbols without including the header that provides them, which
only compiled because the precompiled header supplied it. Every include
the job names is added, in each file's existing include style ("../" in
the GCode/ and Support/ subdirectories, quoted libslic3r/ paths in the
GUI and tests). No code changes.

Verified with scripts/clang_tidy_diff.py -p build-tidy --base eb5b9a77b9
(SLIC3R_PCH=OFF compile database, clang-tidy 22.1.8): no findings left.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L6Kg5igmmMU2YLoK6HrsWV

tests/fff_print/test_precise_seam.cpp

@@ -23,9 +23,11 @@
 #include "libslic3r/GCode/SeamPlacer.hpp"
 #include "libslic3r/GCodeReader.hpp"
 #include "libslic3r/TriangleMesh.hpp"
+#include "libslic3r/ExPolygon.hpp"
 #include <cstddef>
 #include <string>
 #include <string_view>
+#include <cstdlib>
 
 using namespace Slic3r;