Add the includes the precompiled header was supplying on macOS

A build without SLIC3R_PCH had never been tried on macOS. Three files
used what pchheader.hpp happened to include: LocalesUtils.cpp needs
<sstream> and <iomanip>, and the two dialogs need <wx/tooltip.h>. The
GTK port's headers and libstdc++ pull these in transitively, the Cocoa
port's headers and libc++ do not.

src/libslic3r/LocalesUtils.cpp

@@ -3,6 +3,8 @@
 #ifdef _WIN32
     #include <charconv>
 #endif
+#include <iomanip>
+#include <sstream>
 #include <stdexcept>
 
 #include <fast_float/fast_float.h>