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.

src/slic3r/GUI/GUI_Preview.cpp

@@ -20,6 +20,7 @@
 #include "libslic3r/PrintConfig.hpp"
 #include "libslic3r/ExPolygon.hpp"
 #include <cmath>
+#include <cstdint>
 #if ENABLE_OPENGL_AUTO_AA_SAMPLES
 #include "GUI_Init.hpp"
 #endif // ENABLE_OPENGL_AUTO_AA_SAMPLES