Common Linux qplatformdefs.h should not pull in dlfcn.h

When trying to cross-compile for an embedded Linux system that does not support shared libraries fails (i.e. FLAT/nommu):

the mkspecs/linux-g++/qplatformdefs.h header needlessly includes dlfcn.h. any place in the QT code that actually uses dlopen/etc... type functions already have logic/includes in place. globally including this header is pointless.

— a/mkspecs/linux-g++/qplatformdefs.h
+++ b/mkspecs/linux-g++/qplatformdefs.h
@@ -63,7 +63,6 @@
#include <grp.h>
#include <pwd.h>
#include <signal.h>
-#include <dlfcn.h>

#include <sys/types.h>
#include <sys/ioctl.h>