buildsys: fix pregen target (!NPTL with LOCALE)

buildsys: fix pregen target (!NPTL with LOCALE)

When NPTL is not enabled, and LOCALE support is used, compilation fails
with the following error:

-------------------------------------------------------------------------
  MKDIR include/bits
  GEN include/bits/sysnum.h
make[1]: *** No rule to make target `../..//include/bits/uClibc_ctype.h', needed by `../../extra/locale/gen_wc8bit.c'.  Stop.
make: *** [pregen] Error 2
-------------------------------------------------------------------------

The problem is that "headers" is not listed as prerequisite of pregen.
These have been changed multiple times recently:

	o	Austin: order headers before pregen-headers
		1f5c73c9f8a98b1d2a35cea868a585c97ab0e436
	o	Khem: pregen: Fix the parallel build problem in pregen target
		046035ecde92262d96eff2192ba3cda716f04909
	o	Khem: pregen-headers: Add new target which depends on headers.
		ef18cfe8ebab25f5ef92e81956f50e2dc57df602
	o	Carmelo: build: Fix infinite loop when no threading support is enabled
		ff5e4de7088fe8d34812c2a1e604bf04be713606
In all of these commits headers was actually as prerequisite

In commit 9381d622e2411a35a5fd73a5a573eb269e2dd9c9 [nptl: fix buildsys] by Bernhard, headers was removed,
causing the problem reported above.

Signed-off-by: Filippo Arcidiacono <filippo.arcidiacono@st.com>
Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>