loader: include dlfcn.h when building with HAVE_LIBUDEV

loader: include dlfcn.h when building with HAVE_LIBUDEV

The code depending on the definitions is already wrapped
in the same conditional so go ahead and wrap the include.

Otherwise we'll brake compilation on platforms that are
missing the header. Add assert.h in there as well, as it
is introduced and used in the same fashon.

Cc: Eric Anholt <eric@anholt.net>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74122
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>