buildsys: headers target should not depend on sysnum.h

The generation of the sysnum.h has to be a prerequisite of
'pregen' target instead of headers, because it needs the compiler
already available.
The so called C bootstrap headers are required to build the compiler
by invoking 'make headers', so the 'headers' target cannot relies itself
on the compiler to be invoked.

Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>

Makefile.in

@@ -158,11 +158,12 @@ headers_clean-y += HEADERCLEAN_common
 # libc/sysdeps/linux/Makefile.commonarch to headers-y
 headers-y += $(target-headers-sysdep)
 
-headers: $(top_builddir)include/bits/uClibc_config.h $(top_builddir)include/bits/sysnum.h | subdirs
+headers: $(top_builddir)include/bits/uClibc_config.h | subdirs
 subdirs: $(addprefix $(top_builddir),$(subdirs))
-pregen-headers: $(pregen-headers-y)
+pregen-headers: $(top_builddir)include/bits/sysnum.h $(pregen-headers-y)
 pregen: pregen-headers
 	$(Q)$(if $(UCLIBC_HAS_LOCALE),$(MAKE) -C extra/locale locale_headers)
+
 $(top_builddir)include/bits/sysnum.h: $(top_srcdir)extra/scripts/gen_bits_syscall_h.sh | $(top_builddir)include/bits
 	@$(disp_gen)
 	$(Q)set -e; \