Restore the libslic3r Precompiled Header and Direct Includes Lost in the Platform Pass

The platform-file pass treated pchheader.hpp as an ordinary header and
emptied it, and left GUI_Preview.hpp and 14 other files relying on
headers they no longer reached directly.

tests/fff_print/test_gcode_timing.cpp

@@ -12,6 +12,7 @@
 
 #include <cmath>
 #include <cstddef>
+#include <cstdint>
 #include <fstream>
 #include <iomanip>
 #include <ios>