Use the correct expat location

expat.h is (in most Linux distributions, at least) located directly in
/usr/include.  Because the compiler is called with the src directory in
the include path, the include of <expat/expat.h> happens to work but
results in the bundled version of the header being used instead of the
system version.

src/libslic3r/Format/AMF.cpp

@@ -2,7 +2,7 @@
 #include <string.h>
 #include <map>
 #include <string>
-#include <expat/expat.h>
+#include <expat.h>
 
 #include <boost/nowide/cstdio.hpp>